Coating providing panchromatic scattering

ABSTRACT

An article includes a substrate with a surface, and a coating disposed over the surface. The coating includes a binder material and a plurality of porous polymer particles having pores with a variety of pore sizes including a first set of pores having a first average pore size d1 in the range 0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength in the range of 250-400 nm, a second set of pores having a second average pore size d2 in the range 0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength in the range of 400-700 nm, and a third set of pores having a third average pore size d3 in the range 0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength in the range of 700-3000 nm, wherein the porous polymer particles have a shell which is impermeable to a liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 63/126,627, filed Dec. 17, 2020, which is incorporatedherein by reference in its entirety.

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. 63/126,630, entitled: “Coating providing controlledabsorption and scattering”, by K. Lofftus; to commonly assigned,co-pending U.S. patent application Ser. No. 63/126,631, entitled:“Coating providing ultraviolet scattering”, by K. Lofftus; to commonlyassigned, co-pending U.S. patent application Ser. No. 63/126,633,entitled: “Method for fabricating impermeable porous particles”, by K.Lofftus et al.; and to commonly assigned, co-pending U.S. patentapplication Ser. No. 63/126,635, entitled: “Impermeable polymeric porousparticles”, by K. Lofftus et al., each of which is incorporated hereinby reference.

FIELD OF THE INVENTION

This invention pertains to the field of coatings for use in camouflageapplications, and more particularly to coatings providing panchromaticscattering.

BACKGROUND OF THE INVENTION

The purpose of camouflage is to prevent, hinder, or delay the detectionor identification of an object, and in the case of warfare, acquisitionby weapon targeting systems. This is generally accomplished bycontrolling the appearance of the surface of the object so that itblends in with the background. Camouflage uniforms have for many yearsbeen provided to the armed services, to enable soldiers to blend intotheir surroundings and so minimize their risk of being detected by anenemy. Traditional camouflage patterns aim to visually disrupt the thecamouflage generally depends upon the background against which an objectis to be concealed. For flat fields of constant color such as the sky orunbroken snow, the design must reduce the objects contrast with thebackground color thereby reducing the modulation. Background scenes suchas deserts and woodlands have spectral content which varies withwavelength giving rise to colors. There is a modulation patternassociated with each color which can be characterized as a function ofspatial frequency. Camouflage for these types of background scenespreferably matches the spatial frequency characteristics for the colorsin the scene.

Camouflage has traditionally focused on the visible portion of thespectrum, but the use of detection systems that are sensitive to otherwavelength bands (e.g., infrared or ultraviolet) can enable thedetection of camouflaged objects that would be difficult to detectvisually. For example, the Woodland pattern used by the military in theVietnam War has no high near infrared reflectance to emulate greenfoliage, risking exposure of camouflaged objects or persons byhyperspectral analysis.

U.S. Pat. No. 4,611,524 describes camouflage to conceal or delayidentification of a vehicle by matching background visual color on aportion of target. A target becomes detectable when the contrast of thetarget with the background exceeds a certain threshold which will be afunction of the imaging system MTF and noise characteristics. It becomesidentifiable when the contrast for each of a plurality of portions ofthe target is large enough so that the shape of the target can bedetermined.

U.S. Pat. No. 6,805,957 describes a disruptive camouflage pattern systemthat uses specialized techniques for printing the camouflage patternonto fabric. The system provides camouflage in both the human visiblelight and the near infrared range. The system provides a macro patternresulting from a repeat of a micro pattern. The system functions by themacro pattern being disruptive of the subject's shape and the micropattern having sharp edge units of a size capable of blending thesubject into its background.

U.S. Pat. No. 8,307,748 describes camouflage pattern designs to providea range of contrast at multiple scales.

U.S. Pat. Nos. 9,062,938 and 9,074,849 describe camouflage patterns on asubstrate such as a fabric including a set of intermixed coloredblotches selected from a group of colors.

U.S. Pat. No. 3,879.314 discloses aqueous slurries of vesiculatedpolyester resin granules useful as additive colorant in paints

U.S. Pat. No. 5,055,513 discloses compositions suitable for use as acamouflage material based on vesiculated granules of polymer materialcontaining a light reflecting agent. The agent is an oxide, hydroxide orinsoluble salt of magnesium or an insoluble mixed salt of calcium andmagnesium.

U.S. Pat. No. 6,873,283 discloses a camouflage device including acamouflage balloon that can be inflated through an opening to providerapidly deployable camouflage.

U.S. Pat. No. 8,220,379 discloses UV-interactive particles suspended ina binding agent which transmit, reflect, absorb and/or scatterultraviolet rays while being transparent for visible and infraredspectral wavelengths.

U.S. Pat. No. 8,277,876 discloses a camouflage pattern on an item thatemulates the color and UV reflection properties of a landscape patternbut does not teach how to achieve the UV reflection.

Commonly-assigned U.S. Pat. No. 9,891,350, which is incorporated hereinby reference, discloses a light-blocking article having an opacifyinglayer including porous polymeric particles where the opacifying layercomprised 5 to 30% interstitial voids.

U.S. patent application 2014/0261084 discloses a UV reflective pigmentincluding alternating layers of high or low refractive index material.

Japanese patent JP4096760B2 discloses a far-infrared camouflage materialincluding metal thin-film layer, a fine particle-containing resin layer,and a camouflage coloring agent-containing resin layer.

Japanese patent JP5283111B2 discloses a light reflecting material whichis composed of hollow particles constituted of a silicon oxide-basedporous shell having a plurality of micropores.

Most currently-available additive colorants are inorganic metal oxideswith high specific gravities. These colorants have been optimized forreflectivity in the visible range of the electromagnetic spectrum butare sub-optimum for infrared reflectance. Additionally, the betterperforming visible additive colorants absorb in the ultraviolet band ofthe electromagnetic spectrum. Low-cost sensors and cameras areincreasingly available for multispectral imaging in agriculture, art,hunting, and warfare. There is a need to develop colorants that performover a broad range of the electromagnetic radiation spectrum to concealman made targets in a natural environment by better matching the lightabsorption and scattering characteristics of various backgroundsthroughout the infrared, visible and ultraviolet portions of theelectromagnetic spectrum.

SUMMARY OF THE INVENTION

The present invention represents an article including:

a substrate with a surface; and

a coating disposed over the surface, including:

-   -   a plurality of porous polymer particles having pores with a        variety of pore sizes including a first set of pores having a        first average pore size d1 in the range 0.3≤d1/λ1≤0.7, wherein        λ1 is a wavelength in the range of 250-400 nm, a second set of        pores having a second average pore size d2 in the range        0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength in the range of        400-700 nm, and a third set of pores having a third average pore        size d3 in the range 0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength        in the range of 700-3000 nm, wherein the porous polymer        particles have a shell which is impermeable to a liquid; and    -   a binder material.

This invention has the advantage that additive colorants can be providedhaving scattering characteristics that can be controlled across threedifferent wavelength bands, in particular the UV, VIS and IR wavelengthbands.

It has the additional advantage that in some embodiments each of thedifferent pore sizes can be provided in different particles to enableeasier fabrication and control over the relative ratios of the threepore sizes in the coating.

It has the further advantage that the coatings using porous polymerparticles will have a lower weight relative to coatings using additivecolorants made of solid particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a camouflage coating in accordance with the presentinvention;

FIG. 2A is an SEM micrograph of an exemplary porous polymer particlewith an impermeable polymer shell;

FIG. 2B is an SEM micrograph of an exemplary porous polymer particlewithout a polymer shell;

FIG. 2C is SEM micrograph of an exemplary coating including porouspolymer particles with a polymer shell and a film forming latex;

FIG. 3 is a flowchart for a method of fabricating impermeable porousparticles in accordance with an exemplary embodiment.

FIG. 4A illustrates a coating including porous particles having threedifferent pore sizes;

FIG. 4B illustrates a coating including three different types of porousparticles with corresponding pore sizes;

FIG. 4C illustrates a coating including porous particles having twodifferent pore sizes, wherein a third pore size is provided by the voidsbetween the porous particles;

FIG. 4D illustrates a coating including two different types of porousparticles with corresponding pore sizes wherein a third pore size isprovided by the voids between the porous particles;

FIG. 5 is a scanning electron microscope (SEM) micrograph of a porouspolymer particle having three different pore sizes;

FIG. 6A illustrates a coating including porous particles dispersed in abinder with a subtractive colorant;

FIG. 6B illustrates a coating including porous particles having asubtractive colorant disposed on the surface of the pores;

FIG. 6C illustrates a coating including porous particles with asubtractive colorant in an underlying layer;

FIG. 6D illustrates a coating including porous particles with asubtractive colorant in an overcoat layer;

FIG. 7 is a graph comparing measured reflectance spectra for exemplarycoatings;

FIG. 8A illustrates a coating including porous particles with poreswhich scatter radiation in the ultraviolet spectral band;

FIG. 8B illustrates a coating including UV-scattering porous particleshaving a multimodal size distribution to provide a low specularreflectance surface;

FIG. 8C illustrates a UV-scattering coating similar to FIG. 6A with anunderlying layer including porous particles to control scattering andabsorption characteristics in other portions of the spectrum;

FIG. 8D illustrates a low specular reflectance UV-scattering coatingsimilar to FIG. 6C with an underlying layer including porous particlesto control scattering and absorption characteristics in other portionsof the spectrum; and

FIG. 9 illustrates a multimodal particle size distribution.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to the various components ofparticles, coatings, layers and the like is not limiting. Unlessotherwise indicated, the singular forms “a,” “an,” and “the” areintended to include one or more of the components (that is, includingplurality referents). Unless otherwise explicitly noted or required bycontext, the word “or” is used in this disclosure in a non-exclusivesense.

Each term that is not explicitly defined in the present application isto be understood to have a meaning that is commonly accepted by thoseskilled in the art. If the construction of a term would render itmeaningless or essentially meaningless in its context, the termdefinition should be taken from a standard dictionary.

The use of numerical values in the various ranges specified herein,unless otherwise expressly indicated otherwise, are considered to beapproximations as though the minimum and maximum values within thestated ranges were both preceded by the word “about”. In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as the values within the ranges.In addition, the disclosure of these ranges is intended as a continuousrange including every value between the minimum and maximum values, andunless otherwise indicated, the range end points as well.

As used herein, the term “reflectance” refers to the fraction ofincident electromagnetic radiation that is reflected at an interface,for example at a coating interface.

It is an objective of the present invention to provide lightweightadditive colorants for the ultraviolet (e.g., UVA, UVB), visible,infrared (e.g., NIR and SWIR) wavelength bands in the form of porousparticles (e.g., porous polymer particles). In some embodiments, theparticles provide ultraviolet and infrared reflectance for a givenvisible color that are characteristic of the natural environment whencombined with the appropriate subtractive colorants. Some embodiments ofthe invention provide a broad-spectrum match to daylight sky usingporous particles in atmospheric emulation coatings.

The subject matter of this patent relates to the scattering ofelectromagnetic radiation in various portions of the electromagneticspectrum. Within the context of the present disclosure, the followingterminology will be used to refer to various wavelength bands (alsoreferred to synonymously as “spectral bands”). The ultraviolet (UV)wavelength band includes radiation having wavelengths in the intervalfrom 10 to 400 nm. The portion of the UV wavelength range that is ofprimary interest in camouflage applications is 250 to 400 nm. UVA (315to 400 nm), UVB (280 to 315 nm) and UVC (100 to 280 nm) are designationscommonly used to refer to portions of the ultraviolet wavelength band.The visible wavelength band includes radiation having wavelengths in theinterval from 400 to 700 nm. The near-infrared (NIR) wavelength bandincludes radiation having wavelengths in the interval from 700 to 1000μm. The short-wave infrared (SWIR) wavelength band includes radiationhaving wavelengths in the interval from 1000 to 3000 μm. The mid-waveinfrared (MWIR) wavelength band includes radiation having wavelengths inthe interval from 3000 to 8000 μm. The long-wave infrared (LWIR)wavelength band includes radiation having wavelengths in the intervalfrom 8000 to 15000 μm. The far infrared (FIR) wavelength band includesradiation having wavelengths in the interval from 15000 to 1000 μm.

The term colorant refers to any material or addenda added to acomposition for the purpose of modifying the reflectance. Colorants maybe neutral (i.e., achromatic) such that they modify the reflectance ofthe composition in a substantially uniform manner over a spectral regionof interest, or colored (i.e., chromatic) such that the reflectancevaries as a function of wavelength within the spectral region ofinterest.

Colorants can be additive, subtractive, or both additive andsubtractive. Additive colorants reflect light in the spectral region ofinterest thereby adding light to that reflected from an underlyingsubstrate. The reflectance may be achieved by light scattering,diffraction, or constructive interference in a structural colorant. Thereflectance characteristics will typically vary as a function ofwavelength and are generally related to the ratio between the size ofthe colorant particle and any internal structures (e.g., pores) to thatof the wavelengths in the spectral region of interest. Subtractivecolorants remove light by absorption of light passing through a coating,and thus subtract light from that reflected by the substrate. Theabsorption characteristics of the colorant will typically vary as afunction of wavelength within the spectral region of interest, therebycontrolling the color of the reflected light. A subtractive colorant issaid to be a broad-band colorant when it absorbs strongly acrossmultiple wavelength ranges of interest.

Some additive colorants scatter light due to a mismatch between therefractive index of the colorant particles and that of the materialsthat they are dispersed within. Commonly used scattering colorants forthe visible spectrum include titanium dioxide and zinc oxide. A mismatchin the refractive index of air to the solid matter of porous particlescan also act as an additive pigment such that the air-filled pores canbe thought of as an additive colorant particle.

The optimum size of the additive colorant particles and pores is relatedto the first maximum in the Mie scattering while taking into account thenumber of scattering sites per unit volume at a specified porosity. Forporous polymer particles having a porosity less than 35% where the poresare much smaller than the particle and the effective medium refractiveindex is estimated by the Maxwell-Garnett equation the peak Miescattering occurs when the ratio of the pore diameters d to thewavelength is in the range 1.0<d/λ<1.4. At higher porosities, the lowereffective refractive index shifts the peak scattering toward larger porediameters. This shift is offset by the decreasing size of the polymerstructures between pores that scatter light to high angles.

The greatest amount of light is removed from transmission by scatteringat the peak wavelength for Mie scattering. However, much of the light isscattered in the forward direction and the reflected light from additivecolorants is better characterized by light that is scattering in theback direction using the Kebelka-Munk scattering coefficient S. Thiscoefficient may be estimated from Mie theory by:

$S = \frac{C_{sca}\left( {1 - g} \right)}{v}$

where C_(sca) is the scattering cross section of the pore or particlecomputed from Mie theory, g is an asymmetry parameter, and ν is thevolume of the scattering entity. The asymmetry parameter g is definedby:

$g = {\left\langle {\cos(\theta)} \right\rangle = {\frac{1}{k^{2}C_{sca}}{F\left( {\theta,\phi} \right)}{\cos(\theta)}{\sin(\theta)}d\;\theta\; d\;\phi}}$$g = {\left\langle {\cos(\theta)} \right\rangle = {\frac{1}{k^{2}C_{sca}}{\int{\int{{F\left( {\theta,\phi} \right)}{\cos(\theta)}{\sin(\theta)}d\;\theta\; d\;\phi}}}}}$

where k=2π/λ, F(θ,ϕ) is a dimensionless phase function computed from Mietheory, θ is the angle in the scattering plane from forward direction tothe observer, and ϕ is the angle of rotation around the axis of theincident light path. The maximum scattering coefficient S occurs at d/λof about 0.5 when taking into account the number of pores per unitvolume of particle at a specified porosity. This ratio decreasesslightly with increasing porosity. For 50% porous polymer particles, themaximum S occurs at a d/λ ratio of 0.45. The peak is broad with half thepeak height value at d/λ ratios of 0.22 and 1.4.

For longer wavelength bands, the optimum size of the scattering site dbecomes larger and for solid additive colorants, many of which have ahigh specific gravity, this contributes additional weight to thecoating. Coatings using porous particles having pores filled with airhave the advantage that they will have a lower weight to provide theequivalent scattering effect.

Within the context of the present disclosure, the term “light scatteringeffective pore size” is defined to be the size of a mono-sized porehaving the same scattering into the back hemisphere at a specifiedwavelength of light as that of the distribution of pores having the sameat the same porosity.

Within the context of the present disclosure, the term “scatteringopacity” is defined to be the ability of an opacifying layer to preventthe transmission of electromagnetic radiation in a specified wavelengthrange due to scattering in the back direction and is related to Miescattering by the asymmetry parameter. Within the context of the presentdisclosure, the term, “Transmittance” is 100% minus the “scatteringopacity.”

Additive colorants typically have broader spectral peaks thansubtractive colorants, but they may be scattering in some bands andtransparent or absorbing in other bands of interest. For example,titanium dioxide particles that have been optimized for use in paintstypically act as a subtractive colorant in the ultraviolet wavelengthband, as an additive colorant in the visible wavelength band, and arenearly transparent in the longer wavelength infrared wavelength band.Monodispersed particles will produce multiple scattering wavelengthmaxima to produce additional multispectral colorant effects.

Multispectral requirements may require the use of additive colorantsthat are transparent in a wavelength range different from the wavelengthrange where the colorants are needed, for example a wavelength rangewhere a low reflectance is achieved through a subtractive colorant.Another example is where the desired reflectance is achieved in anunderlying layer. An additive colorant may be transparent to wavelengthλ₀ if d/λ₀<0.2 and preferably d/λ₀<0.1 where the pores of size d areuseful as an additive colorant for wavelengths less than λ₀/2. Underthis condition, the pores are too small to scatter a significant amountof radiation at wavelength λ₀ for coating levels that are effective atscattering radiation at wavelengths smaller than λ₀/2. An additivecolorant may be effective at wavelengths larger than λ₀ provided thatd/λ₀>5 and preferably d/λ₀>10. Under this second condition, much of thelight scattered from the additive colorant is scattered in the forwarddirection and contributes little to the reflectance. When this secondcondition is met, the additive colorant is useful for wavelengthsgreater than 2λ₀.

Another form of additive colorant is a structural colorant whereconstructive and destructive interference amplifies or eliminatesscattering to produce a narrower wavelength range of reflectance than apurely scattering colorant. This narrower wavelength range is obtainedby orienting large flake pigments with the flake having a lowerrefractive index core (e.g., amorphous silicon dioxide) and a higherrefractive index coating (e.g., titanium dioxide) where the core andcoating thicknesses are designed to obtain constructive interference ata desired wavelength normal to the surface. The relationship of the coreand coating thicknesses is given by diffraction theory and producesmultiple scattering wavelength maxima to produce additionalmultispectral colorant effects.

Additive colorants can be used amplify the absorption of light bysubtractive colorants. The additive colorant can increase the opacityand hiding power of a coating containing a subtractive colorant. Thisresults from electromagnetic radiation passing through the coating beingscattered by the multiplicity of interfaces with refractive indexdiscontinuity between the different phases in the colorant and withmatrix polymer. The back scattered electromagnetic radiation can againbe scattered and returned in the direction of the incidentelectromagnetic radiation. The path of the light through the coating isincreased by the multiple scatterings thereby increasing the probabilitythat the light is absorbed by the subtractive colorant.

Fluorescent dyes and pigments may also be considered combinationmultispectral colorants when ultraviolet is a wavelength band ofinterest. The absorption of UV light is a subtractive colorant effectwhile the emission of visible light is an additive colorant effect.Similarly, some fluorescent substances absorb blue visible light andemit in the near-infrared band.

Unless otherwise indicated, the term “porous particle” is used herein torefer to polymeric materials useful in the multispectral camouflagecoating compositions essential for the present invention. The porousparticles generally comprise a solid continuous polymeric particlehaving an external particle surface and internal regions absent ofsolids that form pores when dried. The pores may be discretecompartments dispersed within the continuous solid phase isolated bypolymer walls, interconnected compartments absent continuous walls, ornetworks of space between connected a network of polymer nodules.

Unless otherwise indicated, the term “non-porous” is used herein torefer to particles that are not designed to have discrete compartmentswithin the solid continuous polymeric phase. Preferably less than 5% ofthe total volume of the non-porous particles consists of pores.

The polymeric material of the porous particles is generally non-porousand has the same composition throughout that phase. That is, thepolymeric material is generally uniform in composition including anyadditives (for example, colorants or additives) that can be incorporatedtherein. In addition, if mixtures of polymers are used in the polymericmaterial, generally those mixtures are dispersed uniformly throughout.

The term “porogen” refers to a pore forming agent used to make porousparticles for use in the present invention. In some embodiments, theporogen can be the aqueous phase of water-in-oil emulsions (that is thefirst aqueous phase of a water-in-oil-in-water emulsion), the porestabilizing hydrocolloid, or any other additive in the aqueous phasethat can modulate the porosity of the porous particles. In otherembodiments, the porogen can be a diluent that is a solvent or mixturesof solvents for monomers but is not a solvent for the polymer formed inthe emulsion polymerization type of limited coalescence process.

The term “size” as used herein to refer to particles or to internalpores within the particles corresponds to the modal or average diameterof the particles or the internal pores.

The porous particles can include “micro”, “meso”, and “macro” pores,which according to the International Union of Pure and AppliedChemistry, are the classifications recommended for pore sizes of lessthan 2 nm, from 2 to 50 nm, and greater than 50 nm, respectively. Insome configurations, porous polymer particles having two or moredistinct pore morphologies can be used. For spherical pores, thisaverage pore size is an “average diameter”. For non-spherical pores, theaverage compartment size refers to the “average largest dimension.”Average pore size can be determined by analyzing scanning electronmicroscopy (SEM) images of porous particles prepared by freeze fractureor cryo-face-off sectioning using a commercial statistical analysissoftware package to study the distribution of the pores within theparticles, or by manually measuring the pore diameters using the scalein the SEM images. For example, the average pore size can be determinedby calculating the average diameter of at least 200 measured pores in asingle porous particle. The Saltykov method may be applied to correctthe particle size when the fracture or cryo-face-off section surfaceforms a plane and the pores are spherical. Alternately, the spectralresponse of the reflectance of a coating may be fit to one or moreeffective pores sizes using the Mie estimate of the Kebulka-Munkscattering parameter S described above and is preferred for opennetworks of pores.

The porous particles used in this invention preferably have porosity ofbetween 10% and 60%, or more preferably between 10% and 50%, all basedon the total porous particle volume. Porosity can be measured by thewell-known mercury intrusion technique for more brittle polymers wherecomplete crushing of the porous particle occurs before the maximumpressure is achieved. The porosity of particles not fully crushed bymercury intrusion can also be evaluated from the apparent density of theparticles. One method is to measure the buoyancy point using differentdensity liquids, provided that the liquids do not penetrate the particleand fill the pores. Another measurement method is to determine theapparent density from the drag coefficient at a given particle size.This can be done by sedimentation techniques in liquids provided thatparticle in impervious to the liquid. The drag coefficient can also bedetermined in a gas. One useful device is the Aerosizer DSP ParticleSizer Analyzer 3225 which was formerly made by TWI, Inc. of Shoreview,Minn., in which the apparent density applied to the measurement isadjusted so that the modal sizes form this device matches that of onemeasure the size using a device such as a Coulter Counter.

The term “matrix polymer” is used herein to refer to polymers that arepresent in coating formulations (and opacifying layers) that hold theporous particles within the dried layer and give it integrity andflexibility.

The term partition coefficient “Pa/b” refers the ratio of concentrationof a compound in phase a to that in phase b in a mixture of twoimmiscible solvents a and b when at equilibrium.

The term “Log Po/w” is the logarithm base 10 of the concentration of amonomer in octanol to that in water at equilibrium.

The modulation transfer function (MTF) of an optical system describesthe fraction of the modulation or contrast amplitude preserved by theoptical system as a function of spatial frequency.

Camouflage Coatings

Embodiments of the present invention are useful to provide camouflagesurfaces having controlled scattering characteristics in specifiedwavelength bands in order to blend in with various types of backgrounds.FIG. 1 illustrates an article 100 including a camouflage coating 120.While the camouflage coating 120 can be “free-standing” and used as theonly layer or structure in the article, in many embodiments, thecamouflage coating is disposed over a surface 110 of a substrate 105.The coating 120 includes porous particles 125 dispersed within a binder130. The porous particles 125 have pores 140 having specified pore sizedistributions as will be described later. In some exemplary embodiments,the porous particles 125 have an impermeable shell 135 which issubstantially impermeable to liquid (e.g., water), where the term“shell” refers to the external surface of the particle. Theimpermeability is an important attribute for many camouflageapplications because the camouflaged articles are often utilized inenvironments where the surface will be exposed to rain and other sourcesof moisture. Within the context of the present disclosure, the term“substantially impermeable” means that the shell 135 of the particle 125prevents the penetration of the shell by water to cause loss of lightscattering performance for the duration of the mission or the exposureto moisture before a drying event.

In an exemplary embodiment, the porous particles 125 are porous polymerparticles, although other types of porous particles could also be used.In some embodiments, one or more underlying layers 150 can optionally beprovided between the coating 120 and the substrate 105. In someembodiments, one or more overcoat layers 155 can optionally be providedover the coating 120. For example, the overcoat layer 155 can be alow-specular reflectance surface layer such as those described incommonly-assigned U.S. patent application Publications 2020/0199373,2020/0199379 and 2020/0199381 to Lofftus, each of which is incorporatedherein by reference. The article 100 can be any object which it isdesired to camouflage including clothing and military equipment. Thespectral scattering characteristics (i.e., the wavelength dependentscattering characteristics), together with the absorptioncharacteristics of the materials (e.g., colorant materials such as dyesor pigments) in the coating 120 and other layers, provide a camouflagesurface having an appearance that is similar to a specified backgroundwhen viewed visually or using appropriate multispectral imaging systemsin order to mask detection or identification of a camouflaged object.

Porous Polymer Particles

In some exemplary embodiments, the porous particles 125 are porouspolymer particles. For some pore morphologies, the pores aresubstantially spherical and are isolated from other pores within theparticle to form closed or un-networked pores. In an exemplaryembodiment, such pore morphologies are fabricated using an aqueousporogen. The size of the particle, the particle formulation, and themanufacturing conditions are the primary controlling factors for thepore size for the un-networked or closed pore morphology made using awater porogen. Typically, such pores have an average diameter size ofbetween 100 nm and 4 μm, or more typically between 200 nm and 2 μm. Inan exemplary embodiment, such particles can be fabricated using themethods described in commonly-assigned U.S. Pat. No. 7,754,409 to Nairet al., U.S. Pat. No. 7,887,984 to Nair et al., U.S. Pat. No. 7,888,410to Nair et al., U.S. Pat. No. 8,252,414 to Putnam et al., U.S. Pat. No.8,329,783 to Nair et al., and U.S. Pat. No. 9,376,540 to Boris et. al.each of which are incorporated herein by reference. The patents describeporous polymer particles that are made by a multiple emulsion process.The multiple emulsion process provides formation of individual porousparticles having a continuous polymer phase and multiple discreteinternal pores, with the individual porous particles being dispersed inan external aqueous phase. The described Evaporative Limited Coalescence(ELC) process is used to control the particle size and distribution,while a hydrocolloid is incorporated to stabilize the inner emulsion ofthe multiple water-in-oil-in-water (WOW) emulsion that provides thetemplate for generating the pores in the porous particles.

Another method of making polymer particles is known as polymerizationlimited coalescence (PLC) as described in commonly-assigned U.S. Pat.No. 5,563,226 to Muehlbauer et al. and U.S. Pat. No. 9,683,064 toRollinson et al., which are incorporated herein by reference. PLCenables polymer particle formation with compositions and very highmolecular weights that are not soluble in solvents and amenable to theELC process. The toughness of high molecular weight polymer particlesmade by the PLC process may be enhanced by cross-linking withdi-functional and tri-functional monomers.

The PLC method may also be practiced on WOW double emulsions asdescribed in commonly-assigned U.S. Pat. No. 8,703,834 to Nair et al.,which is incorporated herein by reference. Levels of cross-linkingmonomers include 1% to 100% cross-linker. Low levels of cross-linkingproduce solvent swellable particles while high levels produce hard,abrasion resistance particle useful as media for milling pigments asdescribed in in commonly-assigned U.S. Pat. No. 5,478,705, which isincorporated herein by reference. For tough particles, 10% or morecross-linking monomer is required to prevent swelling in solvents whileless than 90% to prevent brittle fracture.

The pores in particles made by a WOW reside uniformly throughout theparticle. Pores near the surface have thin walls that are susceptible tobreakage allowing penetration of the pore by water and resulting in theloss of light scattering functionality useful in color and opacifyingapplications. Additionally, thin walls may be penetrated by moleculardiffusion of water through the free volume of the polymer slowly fillingthe pores with water and resulting in the loss of light scatteringfunctionality. The effect is amplified in high porosity particles wherethe walls both on the surface of the particle and between particles havebeen stretched to the point of breaking during the dilution step beforeevaporation of polymerization. This deficiency is addressed by acore-shell morphology with a shell having much greater thickness thanforms between the pores within the core and the surface of the porousparticle. Commonly-assigned U.S. Pat. No. 8,940,362 to Massa et al.,which is incorporated herein by reference, discloses a ELC WOW method ofcollapsing the pores at the surface to create a non-porous shell aroundporous core. It relies on using a second organic solvent that ismiscible with both water and the first solvent but is not a solvent forthe polymer. This second organic solvent is added to the second waterphase and ELC WOW after the second emulsification step but beforeremoving the solvent. This approach cannot be used for the PLC WOWmethod since no polymer has formed until after polymerization hasoccurred and there is not a first organic solvent to dissolve polymerformed during polymerization.

Another type of pore morphology is formed in particles that are composedof a network of substantially spherical nano-scale sub-particles whichconnect together in random, or pseudo random, chains. The openingsbetween the sub-particles form an open network of interconnecting pores.In an exemplary embodiment, particles with such pore morphologies can befabricated using an organic porogen using the process described incommonly-assigned U.S. Pat. No. 6,726,991 to Keading et al., which isincorporated herein by reference. This patent discloses porous polymerparticles formed using a diluent solvent in the oil phase acts as aporogen in a diluent-oil-in-water (DOW) emulsion using a polymerizationlimited coalescence (PLC) process. The porous polymer particlesdescribed in this patent have a core shell architecture where the shellis an inorganic stabilizer and the porous particles having a mediandiameter of less than about 50 μm.

It is found that particle sizes larger than about 50 μm can be madeusing a second stabilizer poly(2-ethyl-2-oxazoline) as disclosed incommonly-assigned U.S. Pat. No. 9,683,064 to Rollinson et al., which isincorporated herein by reference. It is also found that a polymer shellin a core-shell morphology as described in U.S. Pat. No. 6,726,991 isformed for certain monomers. For these monomers, an impermeable polymershell is formed on a tough, high molecular weight, and optionallycross-linked porous particle. The shell thickness is governed by themonomer and diluent solubilities in water. Increased shell thicknessesare obtained for increasing solubility of the monomer in water. Forexample, styrene and divinyl benzene have very low water solubilitiesand no polymer shell forms while a polymer shell that is about 5 nm inthickness forms for the slightly water-soluble (1.5%) monomer methylmethacrylate (MM) at 50% with 50% trimethylolpropane triacrylate(Tmpta). The water solubility of ethyl acrylate monomers is slightlymore than MM monomer while that of methyl acrylate (MA) is about 5%.Polymer shells of about 5 nm form for 50% ethyl acrylate 50% Tmpta andpolymer shells of about 10 nm form for 50% MA 50% Tmpta. The polymershells are non-porous for particles made with methyl and ethyl acrylatesproviding resistance to loss of performance when pores become filledwith water (e.g., when exposed to rain).

FIG. 2A shows an SEM micrograph of a cryo-face-off section of porousparticle 125 having an impermeable shell 135 (corresponding to PorousParticle P10 described below). The illustrated porous particle 125 is a35% porous particle having 50% MA and 50% Tmpta, where the diluent was5% ethyl acetate and 95% cyclohexane and the colloidal silica is mostlylost during the cyro-face-off sectioning. FIG. 2B shows an SEMmicrograph of a cryo-face-off section of a comparative porous particle125 having no impermeable shell (corresponding to Porous Particle P11described below) made using the process described in U.S. Pat. No.6,726,991. The illustrated porous particle 125 has 40% styrene and 60%divinyl benzene with 55% difunctional activity made by the DOW PLCmethod at 1-to-2 monomer-to-diluent ratio where the diluent comprised25% cyclohexanol and 75% toluene.

The octanol water partition coefficient Po/w, a commonly reportedproperty for a monomer, is closely related to its water solubility andmore closely predicts the propensity of a monomer to form a polymershell in the limited coalescence process. Log Po/w values below 2.0 andpreferably below 1.5 for the majority monomer of the startingcomposition are useful for creating polymer shells. Useful monomers forforming polymer shells in PLC include methyl methacrylate, methylacrylate, ethyl acrylate, 2-hydroxyethyl acrylate, hydroxymethylmethacrylate, 1-hydroxyethyl methacrylate, 2-hydroxyethyl methacrylate,1-hydroxypropyl methacrylate, 2-hydroxypropyl methacrylate,3-hydroxypropyl methacrylate, 1-hydroxybutyl methacrylate,2-hydroxybutyl methacrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutylmethacrylate, methyl 2-(methoxymethyl)acrylate, 2-methoxyethyl acrylate,2-ethoxyethyl acrylate, methyl 2-(hydroxymethyl)acrylate. Usefulcross-linkers with Log Po/w below 2 include ethylene glycol diacrylate,1,3-proanediol diacrylate, and ethylene glycol dimethacrylate.

The polymer shell thickness can be increased using a diluent that hassome water solubility. The water solubility of the monomer is enhancedby the presence of a solvent in the water phase. For example, ethylacetate is soluble in water at about 8% and lowers Log Po/(w+solvent)for the PLC system.

The diluent solvent may be a single solvent or a combination ofsolvents. It has been found that combinations of polar and nonpolarsolvents can provide a network of open meso pores to scatter UV and blueradiation interspersed within low density regions of polymer (or openvoids) that are generally spherical in shape and act as macro pores toscatter VIS, NIR and SWIR radiation. The size and quantity of the macropores decreases with decreasing nonpolar solvent and may be eliminatedaltogether to prove a UV scattering additive colorant.

To be impermeable, the composition of the polymer shell must alsoprevent diffusion of water through the polymer. All amorphous polymersexhibit the property of free volume, the space between randomly packedpolymer chains through which small molecules may diffuse. The freevolume is essential frozen in polymers below glass transitiontemperatures T_(g) and diffusion will be slow. The transition for theglassy state below T_(g) to the rubbery state above T_(g) typicallyoccurs over a range of 10° C. In the rubbery state, the polymer chainsare moving due to the thermal vibrations and the free volume becomesmobile, allowing rapid diffusion of small molecules. To be impermeableto water, the polymer T_(g) onset must be above the operatingtemperature of the coating and there should be no hydrogen bonding.

Depending on the composition of the polymer and small molecule, theremay be an attractive force that allow the particle to absorb the smallmolecule creating a solvent effect. The absorbed molecule separates thepolymer chains allowing movement by thermal energy and effectivelylowering the T_(g) and rapid diffusion of the small molecule through thepolymer. For water, the greatest attractive force is hydrogen bonding,and polymer compositions that do not hydrogen bond provide the greatestimpermeability to water.

Binder Material

The multispectral camouflage coatings of this invention also includes abinder 130 in which the porous particles 125, and optional tintingcolorants are dispersed. In an exemplary embodiment the binder materialis a matrix polymer. In some embodiments, the matrix polymer may includea mixture of polymers. The matrix polymer can be chosen so that it isflexible for applications where the substrate must bend such as clothingor laminates to be applied to curved surfaces. For application on hardsurfaces, the matrix polymer may be hard to increase the coatingdurability.

It is particularly useful that the matrix polymer (i.e., the bindermaterial): (a) is durable and either flexible or hard depending upon theapplication; (b) is capable of forming a stable coating composition withthe porous particles, colorants and any other additives such as UV lightstabilizers; (c) is capable of being coated by techniques practiced inthe art; (d) has film-forming properties when applied to a substrate;(e) is capable of being dried and where desired also crosslinked; (f)has good light and heat stability; and (g) provides additional barrierto penetration of porous particles by liquids.

In some embodiments, the matrix polymer provides an article on asubstrate that has good durability to laundering and can be tailored tosize by sewing. Additionally, the matrix polymer can provide a supplefeel to touch referred to as good hand and flexibility including drapeespecially when coated over a fabric. In other embodiments, the matrixpolymer provides an article on a substrate that has good durability toabrasion, water barrier properties, and high adhesion to hard surfacesuch as metals and plastics. Thus, the matrix polymer is useful in themultispectral camouflage coating composition for binding together andadhering the porous particles and all colorants onto the substrate.

The matrix polymer can include one or more organic polymers that arefilm forming and can be formed as a suspension or emulsion or insolution. It can include polymers that are not crosslinking and to whichadditional crosslinking agents are not added, or it can include polymerto which crosslinking agents are added and are thus capable of beingcrosslinked under appropriate conditions.

One type matrix polymer that can be used is a thermal set acrylic latexdispersion for water-based coating systems with no unsaturated bonds forgreater resistance to photo-oxidation when exposed to sunlight. Otheruseful matrix polymers include but are not limited, to poly(vinylacetate), poly(vinyl pyrrolidone), ethylene oxide polymers,polyurethanes, urethane-acrylic copolymers, other acrylic polymers,styrene-acrylic copolymers, vinyl polymers, and polyesters, siliconepolymers or a combination of two or more of these organic polymers. Suchmatrix polymers are readily available from various commercial sources orprepared using known starting materials and synthetic conditions. Thematrix polymer can be anionic, cationic or nonionic in total charge. Auseful class of film-forming matrix polymers includes aqueous latexpolymer dispersions such as acrylic latexes that can be ionic ornonionic colloidal dispersions of acrylate polymers and copolymers.Film-forming aqueous latexes suitable for use include acrylic latexes,urethane latexes, styrene-butadiene latexes, poly(vinyl chloride) andpoly(vinylidene chloride) latexes, poly(vinyl pyridine) latexes, andpoly(acrylonitrile) latexes.

Latexes are particularly useful in forming impermeable shells on porousparticles with shells that may have a micro pore structure. Thecolloidal size of the latex polymer is much greater than that of themicro pores in the porous particle shell and the latex polymer isprevented from penetrating and filling the porous particle. Thefilm-forming latex polymer coats the exterior of the particle,increasing the shell thickness and sealing any micro pores. FIG. 2Cshows a freeze-fracture SEM micrograph of an exemplary coating 120showing the enhanced impermeable shell 135 obtained in coatingsincluding porous particle 125 having a polymer shell and a film forminglatex where the matrix polymer to porous particle volume is 1 to 4.

In some embodiments including textile substrates, the matrix polymergenerally has a glass transition temperature T_(g) that is less than 25°C. and more likely equal to or less than 0° C. in order to make the drymultispectral camouflage coating flexible, rubbery, and crack-free. Theglass transition temperature can be determined using known proceduresand such values are already known for many polymers useful in thisinvention. The matrix polymer preferably has adequate flexibility andtensile strength in order to maintain integrity upon handling,especially fabric-based applications where good drape, hand, and othertactile properties are required.

In other embodiments including inflexible or hard substrates, the matrixpolymer may have a T_(g) greater than 30° C. and preferable greater than50° C. for coatings on substrates such as metal and plastics. The highT_(g) of the matrix polymer provide strength, resistant to abrasion, andcreep. Additionally, the high T_(g) of a matrix polymer added to theporous polymer shell as shown in FIG. 2C further increases resistance topenetration by water.

In yet other embodiments, the aromatic content of the matrix polymer andporous polymer particles should be minimized to reduce the number ofunsaturated bonds that undergo photo-oxidation and discoloration whenexposed to sunlight. Matrix polymer with aromatic content may providemechanical or chemical advantages such as scratch resistance and waterimpermeability when included in the topmost multispectral camouflagecoating 120. The aromaticity should not exceed 10% of the combinedcoating 120 and any underlying layers 150 that rely in porous particlesas additive colorants. The aromatic content may be concentrated in thetopmost coating to enhance the surface properties of the multispectralcamouflage coating.

The matrix polymer can optionally be crosslinked with a crosslinkingagent that is included in the multispectral camouflage coatingformulation and which is activated chemically with heat, radiation, orother means in order to provide enhanced integrity and wash durabilityof the resulting article. The crosslinking agent serves to provideimproved insolubility of the multispectral camouflage coating in waterand adhesion to the substrate or optional underlying layer. Thecrosslinking agent is a chemical having functional groups capable ofreacting with reactive sites on the latex polymer under curingconditions to thereby produce a crosslinked structure. Examples ofsuitable crosslinking agents include multi-functional aziridines,aldehydes, and epoxides.

Drying and optional crosslinking of the matrix polymer in themultispectral camouflage coating formulation can be accomplished bysuitable means such as by heating, and various mechanisms can beemployed for crosslinking the matrix polymer. For example, thecrosslinking can involve condensation or addition reactions promoted byheat or radiation. In one embodiment, a latex composition is used as thematrix polymer. Upon heating, the latex film dries, with a crosslinkingreaction taking place between the reactive side groups of the polymerchains. If the particular latex polymer used is not itself heatreactive, then suitable catalysts or crosslinking agents can be added topromote crosslinking upon heating.

In addition, the matrix polymer provides control of the void spacing(interstitial voids or volume) among porous particles. Commonly assignedU.S. Pat. No. 9,891,350 teaches the interstitial void is preferably atleast 5% to provide the greatest light blocking capacity. The size ofthe interstitial voids is defined by the size distribution of the porousparticle and may be as large as 40% of the porous particle for lowinterstitial volumes and greater than the porous particle size for highinterstitial void volume. Such large pores are effective for scatteringof longer wavelengths and may be part of the design in some embodimentsor undesirable in other embodiments where high transmittance at thesewavelengths is needed.

The interstitial voids may become connected above a percolationthreshold of about 5 to 10% allowing moisture vapor and water topenetration the coating. In some embodiments, connected interstitialvoids provide breathability in camouflage garments. Light scattering atthe longer wavelengths is reduced and lost when the connectedinterstitial voids are filled with water. Additionally, theamplification provided by the interstitial voids of the light blockingat shorter wavelengths is lost. Reduced light block of the camouflagecoating 120 resulting in greater transmittance to underlying layer 150and a change in color properties of the camouflage. Furthermore, waterin the interstitial voids expose the internal porous particles to waterwhich may absorb the water unless made impervious by a polymer shellformed on the particle or with the matrix polymer as shown in FIG. 2C.To minimize penetration of the coating by water, it is desirable tolimit the volume of the interstitial voids not filled with binder to beno more than 10%, and preferably to be no more than 5%.

The amount of matrix polymer required to reduce the interstitial voidvolume to less than 5% depends upon the random packing density of theporous particles. The random packing density varies the polydispersityand shape of the porous particle. It is greater for spherical particlesbut less for narrow particle size distributions such as those producedby LC processes. The random packing density of the LC porous particlescan be increased to reduce the amount of matrix polymer required byusing two or more sizes of particles where the ratio of particle sizesis about 2.

The matrix polymer is typically present in the multispectral camouflagecoating in an amount of at least 25 volume % and up to and including 50volume %, or more typically at least 30 volume % and up to and including45 volume %. The weight % of the matrix polymer based on the total dryweight of the multispectral camouflage coating (that is, total layersolids) to obtain the desired volume % depends upon the porosity of theparticles. For example, to obtain 25 volume % matrix polymer, 29 weight% is needed for particles with 10% porosity while 57 weight % is neededfor 60% porosity. For 50 volume % polymer, 55 weight % matrix polymer isneeded for particles with 10% porosity while 80 weight % is needed for60% porosity.

Substrates

The substrates 105 onto which the multispectral camouflage coatings ofthe invention, and optionally one or more underlying layers 150, areformed or disposed can include various woven and nonwoven textilefabrics such as nylon, polyester, cotton, glass, aramide, rayon,polyolefin, acrylic wool and felt, polymeric films, cellulose,polyethylene terephthalate (PET), diacetyl cellulose, acetate butyratecellulose, acetate propionate cellulose, polyether sulfone, polyacrylicbased resin, for example, poly(methyl methacrylate), apolyurethane-based resin, polyester, polycarbonate, aromatic polyamide,polyolefins (for example, polyethylene and polypropylene), polymersderived from vinyl chloride (for example, polyvinyl chloride and a vinylchloride/vinyl acetate copolymer), polyvinyl alcohol, polysulfone,polyether, polynorbonene, polymethylpentene, polyether ketone,(meth)acrylonitrile], adhesive laminates such as 3M™ Wrap Film Series1080, paper or other cellulosic materials, canvases, wood, metals,plaster and other materials that would be apparent to one skilled in theart. The substrates can vary in thickness, suitable for the desiredapplication. Particularly useful substrates comprise a textile web,adhesive laminate, metalized polymer, cellulosic material, glass, orceramic. Textiles and fabrics are useful for articles such as clothing,tarpaulins, and tents, while adhesive laminates are more useful forchangeable camouflage of equipment such as trucks, tanks, and aircraft.

Various substrates 105 including polymeric films, adhesive laminates,textiles, and cellulosic substrates can be surface treated by variousprocesses including corona discharge, glow discharge, UV or ozoneexposure, flame, or solvent washing in order to promote adhesion ofcoating compositions.

The thickness of the substrate 105 is not critical and can be designedfor a given use of the resulting article. In typical embodiments, thedry substrate thickness is at least 50 μm.

Underlying Layers

Depending upon the particular application being considered, the article100 of the present invention can further include one or more underlyinglayers 150. In some embodiments, the underlying layers 150 can includeone or more of a multispectral base color layer, a coloration patternlayer, a barrier layer, a thermal management layer, or a primer layer,and an adhesion-promotion layer to promote the adhesion of themultispectral camouflage coating 120.

A multispectral base color layer can be used to provide additive orsubtractive coloration in spectral bands where the camouflage coating120 is transparent. The base color must be compatible with thecoloration of any coloration pattern layers in the spectral range wherethe colorant contained therein are active in absorbing or scatteringelectromagnetic radiation. For example, a broad band black subtractivecolorant base layer comprising carbon to emulate deep space can be usedunder an additive colorant layer to emulate atmospheric scatteringuseful for camouflage of aircraft in daylight.

One or more barrier layers can be used to prevent migration of substratecomponents that act as subtractive colorants into any overlying layers.The T_(g) and composition of the barrier layer must be chosen to preventdiffusion of the substrate component over the expected operatingtemperature. If applied over the base colorant layer, it must betransparent to wavelength bands where that base colorant is to provide adesired reflectance in the overall camouflage scheme. Barrier layers mayalso act a primer layer. For example, the barrier layer can be a barrierto UV absorbing oils added to vinyl laminate substrates to impart lightstability to prevent polymer degradation by photo-oxidation when exposedto the UV components of sunlight.

In some embodiments, a coloration pattern layer can be a colorantreceiver layer which is adapted to receive water-based inks. Forexample, the coloration pattern layer can be a coating on an appropriatefabric for print-on-demand camouflage printed with either continuous ordrop-on-demand ink jet. In some embodiments, the colorant receiver layercan be the coating 120 which includes the porous particles 125 or can bean overcoat layer 155.

Subbing compositions for primer layers to promote the adhesion are wellknown in the art and any such compositions can be used. Some usefulsubbing compositions include but are not limited to polymers derivedfrom vinylidene chloride such as vinylidene chloride/methylacrylate/itaconic acid terpolymers and vinylidenechloride/acrylonitrile/acrylic acid terpolymers. These and othersuitable subbing compositions are described in numerous publications andwell-known in the photographic film coating art. A polymeric subbinglayer can additionally be overcoated with a second subbing layercomprised of a gelatin (typically referred to as a “gel sub”).

In some embodiments, primer layers with primer compositions that includeelectro-oxide under corona discharge treatment (CDT) can be used.

An adhesion-promotion layer can be disposed between the substrate 105and the multispectral camouflage coating 120 to improve adhesion betweenthe two materials, especially if the substrate 105 is a flexible textilematerial. The adhesion-promotion layer can be any material thatmaintains its flexibility and integrity and prevents cracking upondrying such as described above for the matrix polymers and in oneembodiment can be the same as the matrix polymer used in the overlyingopacifying layer. Particularly useful polymeric materials useful forforming the adhesion-promotion layer are polymers that provide theunderlying layer with a glass transition temperature below 15° C. andpreferably below that of the coldest temperature the at which the goodflexibility is required. For example, such useful polymeric materialsinclude but are not restricted to acrylic polymers, styrene-acryliccopolymers, vinyl polymers, polyurethanes, silicones, or a combinationof two or more of these polymers, rubbers and latexes made from1,3-butadiene, including, but not limited to styrene butadiene,polybutadiene, polychloroprene (Neoprene) and nitrile rubbers. Examplesof suitable commercially available polymers for the underlying layer arethose sold under the tradenames Butonal® NS175 (BASF) and Hystretch V43®(Lubrizol Corp.).

In some embodiments, coating layers may be treated such that they aremade insoluble to subsequent coating vehicles by cross linking thepolymer in the layer.

The layer formulations described herein can contain additives such asflame retardants, light stabilizers, preservatives, antimicrobials,biocides, surfactants, defoamers, and leveling and pH control agents, inorder to achieve the desired properties of the layer formulations forapplication to the substrate or underlying layers provided the additivesdo not act as subtractive colorants at wavelengths where highreflectance is desired. It is desirable that the various layerformulations have good wetting and film-forming properties. Materialssuch as silicones can be incorporated into the formulations to aid inleveling them on the surface of the substrate or any underlying layer toprovide a smooth finish.

Overcoat Layers

Various types of overcoat layers 155 can be useful in differentembodiments. The overcoat layers can provide various functions such asadded durability and control of specular reflections.

In some multispectral camouflage applications, the low specularreflectance coatings disclosed in the aforementioned commonly-assignedU.S. patent application Publications 2020/0199373, 2020/0199379 and2020/0199381 to Lofftus are useful as overcoat layers. These coatingsinclude a plurality of protruding substantially spherical caps having amultimodal size distribution 200 (see FIG. 9). In some embodiments, thespherical caps are formed by spherical particles which protrude from thesurface of the overcoat layer. The spherical caps in the modes of themultimodal size distribution have a prescribed ratio of sizes and areacoverage to disperse specular light more uniformly over the hemispherenormal to the surface of the coating. Generally, low specularreflectance is desirable over all wavelength ranges where the objectbeing concealed is exposed to diffuse light source and for specularlight sources that are juxtaposed to the detector from which the objectis to be concealed. In some embodiments, low specular coatings can beprovided over the top of coatings containing the scattering porousparticles of the present invention to control the specular reflectancecharacteristics of the surface together with the absorption andscattering characteristics of the surface.

Fabrication of Impermeable Porous Particles

FIG. 3 shows a flowchart for a method of fabricating impermeable porousparticles in accordance with an exemplary embodiment. The impermeableporous particles are core/shell particles having a porous polymer coreand a polymeric shell where the particle comprises a polymer that isinsoluble in solvents or water owing to the very high molecular weightof the polymer and cross-linking.

In a form suspension of monomer droplets step 300, a suspension ofmonomer droplets is formed in an aqueous medium. In a preferredembodiment, the monomer droplets are ethylenically unsaturated monomerdroplets containing one or more monomers and a porogen, wherein at leastone of the monomers is a cross-linking monomer. A majority of themonomers in the monomer droplets preferably have a log Po/w below 2.0,and more preferably below 1.5.

The porogen functions to create internal porosity in the particles onceit is removed. The porogen comprises one or more solvents for themonomer and acts as diluent for the monomer to define the porosity oncethe monomer has been polymerized. The porogen must be at most onlypartially miscible in water and not be a strong solvent for the polymer.In an exemplary embodiment, the porogen includes two or more solvents,wherein one of the solvents has a greater miscibility in water than theother solvents. For example, the porogen can include cyclohexane andethyl acetate, where ethyl acetate has a greater miscibility in waterthan cyclohexane. Ethyl acetate has a solubility of 8.3% in water at 20°C. and a log Po/w of 0.71 while the solubility of cyclohexane in wateris about 0.005% with an estimated Po/w of 3.4. The partial miscibilityof ethyl acetate enhances the solubility of monomers in the water phaseand aid in the formation of a polymer shell at the oil water interfaceduring polymerization. However, the solubility of low molecular weightmolecules of polymer in ethyl acetate creates micro pores in the polymershell for porogens with high levels of ethyl acetate and requires matrixpolymers that seal the shell against penetration by water. Coatingsusing porous particles made using MM and hexane diol diacrylate (Hdda)using a porogen of 20% ethyl acetate and 80% cyclohexane were penetratedby water in fifteen minutes when coated using a low T_(g) matrix polymerbut not penetrated for 2.5 hours when coated using a high T_(g) matrixpolymer. On the other hand, porous particles made with MM and Tmptausing a porogen of 5% ethyl acetate and 95% cyclohexane were notpenetrated by water in 8 hours when the same low T_(g) matrix polymer isused.

The porogen must be miscible with the monomers before polymerization toensure uniform distribution throughout the suspension droplets allowinga uniform distribution of monomer at the droplet-water interface toproduce a consistent shell thickness. The pore morphology may becontrolled by the solubility of the resulting polymer in the porogen. Asthe molecular weight of the polymer builds past the solubility limit inthe porogen during the polymerization, the polymer precipitates into asolid phase leaving behind meso and macro pores filled with porogen,monomer, oligomers, and lower molecular weight polymer. As the monomersare depleted, polymerization rate slows with less precipitationoccurring and the remaining monomer, oligomers, and lower molecularweight polymer add to the precipitated polymer to build and connect anetwork of polymer nodules to the shell leaving an open network ofinternal pores. The structure of this polymer network and resultingpores may be varied by changing the molecular weight at which polymerprecipitates. For example, the morphology of poly methyl methacrylate(PMMA) can be controlled by the ratio of cyclohexane and ethyl acetatein the porogen. Cyclohexane is a poor solvent for PMMA while ethylacetate is a good solvent for PMMA up to molecular weights of 250,000.Porogens high in cyclohexane will produce very large heterogenous porestructures while porogens with significant ethyl acetate will producesmall uniform pore structures. When the porogen is a strong solvent forthe polymer being made, the uniformity will be maintained throughout thedroplet to the completion of polymerization and only micro pores toosmall to scatter light are formed.

In an exemplary embodiment, the aqueous medium contains a firststabilizer (also known as the particulate dispersing agent) and apolymerization initiator, wherein the first stabilizer is an inorganiccolloid. The first stabilizer may also be an organic colloid asdescribed in commonly-assigned U.S. Pat. No. 4,965,131 to Nair et al.,which is incorporated herein by reference. The aqueous medium optionallycontains a second stabilizer to provide a colloidally stabilizedsuspension. In an exemplary embodiment, the second stabilizer is apoly(2-ethyl-2-oxazoline) homopolymer as described in commonly-assignedU.S. Pat. No. 9,683,064 to Rollinson et al.

In some embodiments, the suspension optionally includes a promoterspecies to assist in the adherence of the particulate stabilizermaterials to the interface between the discontinuous organic monomerphase droplets and the continuous aqueous phase. Low molecular weightpolymers are useful as promoters. For example, oligomeric condensationpolymers of methyl aminoethanol and adipic acid (see commonly-assignedU.S. Pat. Nos. 4,833,060, 4,965,131, 6,726,991 and 7,888,410) whichdrives the first stabilizer to the interface between the water layer andthe polymer solvent droplets.

A polymerize monomers step 310 is used to polymerize the monomers in thesuspension. In a preferred embodiment the polymerization of the monomersis initiated by an initiator that is activated by heating thesuspension. The result of the polymerization monomers step 310 is theformation of core/shell particles having a core of a porous polymer anda polymeric shell. The porous polymer core includes pores having adistribution of pore sizes. The pore sizes can be controlled by theratio of monomer to porogen and by the solubility of the resultingpolymer in the porogen. The polymeric shell preferably has a shellthickness of at least 5 nm and does not contain any pores having adiameter of more than 2 nm. The shell forms from monomer molecules thathave dissolved in the water phase that diffuse back to the dropletsurface and are polymerized at the interface as monomer within thedroplet is depleted by polymerization. Monomers that have somesolubility are water are required with Log Po/w values below 2.0 andpreferably below 1.5 for the majority monomer in the droplet are usefulfor creating polymer shells.

A remove porogen step 320 is used to remove the porogen from thecore/shell particles. In an exemplary embodiment, this step is performedby nitrogen swept distillation above the boiling point of the solventsin the porogen. For impermeable shells, it was found that adding awater-soluble extraction solvent to the water phase accelerated theremoval the porogen. In an exemplary process, isopropyl alcohol (IPA) isadded as the extraction solvent. After polymerization is complete, 100%IPA is added at a ratio of 2 parts to 3 parts water phase and thesuspension heated to 70° C. with stirring for 30 minutes while anitrogen sweep is used to remove extracted solvents. The lost volume ofwater phase is replenished by adding same volume but using 50% IPA inwater and the temperature increased to 80° C. for 30 minutes. Lost waterphase volume is subsequently replenished with water as the suspension isstirred at 90° C. for 1 hour and 95° C. for 3.5 hours.

The porous polymer particles are recovered by filtration followed byrinsing multiple times with distilled water. The particles are kept as awet filter cake to aid dispersion in coating fluids. For good shelflife, the last rinse may include a biocide.

An optional remove inorganic colloid step 330 can be used to remove theinorganic colloid stabilizer. The remove inorganic colloid step 330 maybe performed on the suspension before filtering or rinsing. Alternately,the wet cake may be resuspended in water. In an exemplary embodiment,this step was performed by resuspending wet cake using equal weights ofwet cake and water. To this one eighth part 25% potassium hydroxide wasadded the suspension stirred for 45 minutes. The suspension was filteredand the particles resuspended in 0.1N potassium hydroxide for 30minutes. The suspension was filtered and rinsed until a neutral pHeffluent was obtained.

In accordance with embodiments of the present invention, the resultingcore/shell particles are used to form a coating disposed on a surfacehaving specified scattering and absorption characteristics that areuseful for camouflage applications. This can be done by various methodsincluding those described below in the Coating Formulations and Methodssection. The coating fluid dispersion preferably includes a film-forminglatex. When the coating fluid dries, the film-forming latex forms a filmover the surface of the polymeric shell which causes the shell to beimpermeable to a liquid. Within the context of the present invention, ashell is said to be impermeable if a liquid does not pass through theshell to substantially permeate the porous core within a time intervalof 30 minutes. The porous core is said to be substantially permeated bya liquid when the amount of radiation in an appropriate wavelength bandwhich is scattered by the particles is reduced by more than 50%.

Panchromatic Scattering Coatings

In some camouflage applications it is desirable for the surface of thearticle to be scattering across a wide range of wavelengths (e.g., inthe ultraviolet, visible and near-infrared wavelength bands). Forexample, such coatings would be useful for aircraft which should lookwhite against a cloud-covered sky. FIGS. 4A-4D illustrate severalmultispectral camouflage configurations which provide thischaracteristic.

FIG. 4A illustrates an article 100 having a coating 120 disposed overthe surface 110 of a substrate 105 in accordance with an exemplaryembodiment. The coating 120 includes a plurality of panchromaticscattering porous particles 125 dispersed in a binder 130. Each of thepanchromatic scattering porous particles 125 include pores 140 having avariety of pore sizes. In particular, the pores 140 include: a first setof pores 142 having a first average pore size d1 in the range0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength in the range of 250-400 nm; asecond set of pores 144 having a second average pore size d2 in therange 0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength in the range of 400-700nm; and a third set of pores 146 having a third average pore size d3 inthe range 0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength in the range of700-3000 nm. The first set of pores 142 is adapted to scatterultraviolet light, the second set of pores 144 is adapted to scattervisible light, and the third set of pores 146 is adapted to scatterinfrared light.

FIG. 4B illustrates an article 100 having a coating disposed over thesurface 110 of a substrate 105 in accordance with another exemplaryembodiment. In this case, the coating 120 includes porous particles 125a, 125 b, 125 c each having a different pore size. Ultravioletscattering porous polymer particles 125 a are adapted to scatterultraviolet light and have a first set of pores 142 with a first averagepore size d1 in the range 0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength inthe range of 250-400 nm. Visible scattering porous particles 125 b areadapted to scatter visible light and have a second set of pores 144 witha second average pore size d2 in the range 0.3≤d2/λ2≤0.7, wherein λ2 isa wavelength in the range of 400-700 nm. Near infrared scattering porousparticles 125 c are adapted to scatter near infrared light and have athird set of pores 146 with a third average pore size d3 in the range0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength in the range of 700-3000 nm.

FIG. 4C illustrates an article 100 having a coating disposed over thesurface 110 of a substrate 105 in accordance with another exemplaryembodiment. The coating 120 includes a plurality of polychromaticscattering porous particles 125 dispersed in a binder 130. Each of thepanchromatic scattering porous particles 125 include pores 140 having avariety of pore sizes. In particular, the pores 140 include: a first setof pores 142 adapted to scatter visible light having a first averagepore size d1 in the range 0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength inthe range of 250-400 nm; and a second set of pores 144 adapted toscatter visible light having a second average pore size d2 in the range0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength in the range of 400-700 nm.The polychromatic scattering porous particles 125 are adhered to eachother and to the substrate 105 with a binder 130 which doesn't fill thevoids between the polychromatic scattering porous particles 125. Theunfilled voids between the polychromatic scattering porous particles 125provide a third set of pores 146 adapted to scatter infrared lighthaving a third average pore size d3 in the range 0.3≤d3/λ3≤0.7, whereinλ3 is a wavelength in the range of 700-3000 nm.

FIG. 4D illustrates an article 100 having a coating disposed over thesurface 110 of a substrate 105 in accordance with another exemplaryembodiment. This embodiment includes features from both FIG. 4B and FIG.4C. In this case, the coating 120 includes porous particles 125 a, 125b, each having a different pore size. Ultraviolet scattering porouspolymer particles 125 a are adapted to scatter ultraviolet light andhave a first set of pores 142 with a first average pore size d1 in therange 0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength in the range of 250-400nm. Visible scattering porous particles 125 b are adapted to scattervisible light and have a second set of pores 144 with a second averagepore size d2 in the range 0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength inthe range of 400-700 nm. The porous particles 125 a, 125 b are adheredto each other and to the substrate 105 with a binder 130 which doesn'tfill the voids between the porous particles 125 a, 125 b. The unfilledvoids between the porous particles 125 a, 125 b provide a third set ofpores 146 adapted to scatter infrared light having a third average poresize d3 in the range 0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength in therange of 700-3000 nm.

FIG. 5 shows a cryo-face-off SEM micrograph of an exemplary ELC WOWparticle comprising cellulose acetate butyrate porous particle 125having three different pore sizes corresponding to sets of pores 142,144, 146. Note that the particle and pores in FIG. 5 are spherical andtheir cross sections have been compressed to ellipses by the microtomeforce in the cryo-face-off preparation method.

Colored Camouflage with Infrared Scattering Coatings

In some camouflage applications it is desirable for the surface of thearticle to be scattering in the infrared wavelength band to supplementcolored light reflected from the surface having a color controlled bysubtractive colorants. For example, many conventional camouflage fabricsare colored and patterned such that they match a typical background(e.g., foliage) with the visible wavelength band, but can be more easilydetected within the infrared wavelength band. In an exemplarymultispectral camouflage embodiment, a layer including porous particlescan be used to control scattering in the infrared to more closely matchthe spectral characteristics of the background. FIGS. 6A-6D illustrateseveral multispectral camouflage configurations which provide thischaracteristic.

FIG. 6A illustrates an article 100 having a coating 120 disposed overthe surface 110 of a substrate 105 in accordance with an exemplaryembodiment. The coating 120 includes a plurality of porous particles 125dispersed in a binder 130. Each of the porous particles 125 includepores 140 having a specified distribution of pore sizes. The pores 140include a set of pores adapted to scatter infrared light having anaverage pore size d in the range 0.3≤d/3≤0.7, wherein λ3 is a wavelengthin the range of 700-3000 nm. In some embodiments, the porous particles125 can also include pores adapted to scatter light in other wavelengthbands.

A subtractive colorant 132 is used to control a visible color of thearticle. In the example of FIG. 6A, the subtractive colorant 132 isdispersed in the binder 130. In other embodiments, the subtractivecolorant 132 can be provided in other locations. For example, in theconfiguration of FIG. 6B, particles of colorant 132 can be disposed onthe surface of the pores 140 within the particles 125. This can beaccomplished by adding a dispersion of the subtractive colorant 132 tothe porogen during the porous particle manufacturing. The subtractivecolorant 132 may also be dispersed throughout the polymer of the porousparticle by incorporating an oil dispersion of the subtractive colorant132 to the oil phase of the porous particle manufacturing.Alternatively, the colorant can be distributed within an underlyinglayer 150 positioned between the coating 120 and the substrate 105 asillustrated in FIG. 6C, or in an overcoat layer 155 as illustrated inFIG. 6D.

FIG. 7 shows a graph 240 including measured total diffuse reflectancespectra 250, 260, 270 for normal incidence light. Spectrum 250corresponds to 3M™ Wrap Film Series 1080 matte black vinyl. Spectrum 260corresponds to Inventive Example 3 blade coating on 3M™ Wrap Film Series1080 matte black vinyl comprising a scattering layer with 350 ppm DirectBlue (DB) 71. Spectrum 270 corresponds to Inventive Example 4 spraycoating on a balloon comprising a scattering layer with 1760 ppm DB 71.The level of DB 71 Spectrum 260 is nearly imperceptible at a wavelength600 nm while that of Spectrum 270 is evident to maintain a strong bluecolor while achieving high UVA/B reflectance. The NIR/SWIR reflectanceof Spectrum 270 is higher due to the use of porous particle in theabsorbing underlayer.

Ultraviolet Scattering Coatings

In some camouflage applications it is desirable for the surface coatingof the article to be scattering in the ultraviolet wavelength band,while being transparent in the visible and short-wave infrared bands.For example, when an aircraft is viewed from below against the sky,conventional surfaces can be detected by a lack of radiation in theultraviolet portion of the spectrum. By covering the surface with acoating which scatters ultraviolet light, the surface of the airplanecan appear to more closely match the appearance of the background sky(either cloudy or clear sky) which includes high level of ultravioletradiation. If the coating is transparent in the visible and infraredwavelength bands, the spectral content in these bands can be controlledby the color of absorbing layers or the substrate. In some embodiments,the coating can also be designed to scatter some degree of blue lightwhich can be used to simulate the appearance of blue sky. FIGS. 8A-8Dillustrate several UV scattering camouflage configurations.

FIG. 8A illustrates an article 100 having a coating 120 disposed overthe surface 110 of a substrate 105 in accordance with an exemplaryembodiment. The coating 120 includes a plurality of porous particles 125a dispersed in a binder 130. Each of the porous particles 125 a includepores 140 having a specified distribution of pore sizes. In an exemplaryconfiguration, the pores 140 have a light scattering effective pore sizeof no more than 100 nm. This ensures that the pores will provide a highdegree of scattering in the ultraviolet spectral band without scatteringsignificantly in the longer wavelength visible and infrared spectralbands. Preferably, the coating has a scattering opacity of no more than20% in the wavelength band from 500 to 3000 nm (which includes thegreen, yellow, orange, and red portions of the visible spectrum, as wellas the NIR and SWIR portions of the infrared spectrum. In someembodiments, the porous particles 125 a have an impermeable shell 135,which is impermeable to a liquid as has been discussed earlier.

In some embodiments, the low-specular reflectance surfaces disclosed inthe aforementioned, commonly-assigned U.S. patent applicationPublications 2020/0199373, 2020/0199379 and 2020/0199381 to Lofftus canbe combined with the ultraviolet scattering porous particles of FIG. 8A.In some arrangements, the low-specular reflectance surface can beprovided over the top of the coating 120 as an overcoat layer 155 (FIG.1).

In another arrangement, the porous particles themselves can be used toform the low-specular reflectance surface as illustrated in FIG. 8B. Inthis arrangement, the porous particles protrude from the coating 120 toprovide substantially spherical caps having a multimodal sizedistribution. As described in more detail in the aforementioned U.S.patent applications by K. Lofftus, the multimodal size distribution hasa distribution function having two or more modes, each mode having apeak defining an associated mode cap size, wherein the distributionfunction includes a first mode having a first peak corresponding to afirst cap size and a second mode having a second peak corresponding to asecond cap size. A mode width parameter for each of the modes ispreferably less than or equal to 1.0, the mode width parameter for aparticular mode being given by a ratio of a full-width half-maximumwidth of the particular mode to the cap size of the particular mode. Ina preferred embodiment, a ratio of the second cap size to the first capsize is between 1.7-4.0, a smallest of the mode cap sizes is greaterthan or equal to 1.0 microns, and a largest of the mode cap sizes isgreater than or equal to 3.0 microns. In the exemplary embodiment shownin FIG. 8B, the multimodal size distribution includes three modescorresponding to three particle sizes: a first particle sizecorresponding to porous particles 125 d, a second particle sizecorresponding to porous particles 125 e, and a third particle sizecorresponding to porous particles 125 f. Each type of porous particles125 d, 125 e, 125 f have pores 140 having light scattering effectivepore size of no more than 100 nm such that they scatter ultravioletradiation as discussed earlier. The porous particles 125 d, 125 e, 125 fare applied to the surface 110 of the substrate 105 using a binder 130which is applied in an amount which leaves the upper surfaces of theporous particles 125 d, 125 e, 125 f protruding from the coating toprovide corresponding caps 160, 162, 164.

FIG. 9 illustrates an exemplary multimodal particle size distribution200 useful to form the low-specular-reflectance coating 120 of FIG. 8B.The multimodal particle size distribution 200 is characterized by adistribution function 205 having two modes, and preferably three or fourmodes. The illustrated distribution function 205 includes a first mode210 having a first peak corresponding to a first particle size D₁, and asecond mode 212 having a second peak corresponding to a second particlesize D₂. (As used herein, the first mode 210 refers to the mode havingthe smallest diameter and each subsequently numbered mode is the nextlargest mode in the distribution.) A ratio of the second particle sizeto the first particle size is preferably in the range 1.7≤D₂/D₁≤4.0. Inan exemplary configuration, the ratio of the second particle size to thefirst particle size is about D₂/D₁≈2.0. More preferably, thedistribution function 205 further includes an optional third mode 214having a third peak corresponding to a third particle size D₃, where aratio of the third particle size to the second particle size is between1.7≤D₃/D_(2≤4.0). In some embodiments, the distribution function furtherincludes a fourth mode 216 having a fourth peak corresponding to afourth particle size D₄, where a ratio of the fourth particle size tothe third particle size is between 1.7≤D₄/D₃≤4.0. Preferably, thelargest particle size should be at least 4× the wavelength of the lightbeing diffused based on the performance of measured samples. For visiblelight applications where the larges wavelength is about 750 nm, thelargest particle size should preferably be at least 3.0 microns. Thesmallest particle size should preferably be larger than 0.7× thewavelength of the light being diffused based on Mie scattering theory,and more preferably should be larger than 1× the wavelength. Inpreferred embodiments, the smallest particle size is greater than orequal to 1 micron, and more preferably is greater than or equal to 2microns. In the example of FIG. 8B, the multimodal particle sizedistribution 200 has three modes 210, 212, 214.

FIG. 8C illustrates another exemplary embodiment, which includes acoating 120 similar to that of FIG. 8A. In this case, underlying layer150 is used to control scattering and absorption characteristics inother portions of the spectrum. The underlying layer 150 includespanchromatic scattering porous particles 125 similar to those shown inFIG. 4A. The underlying layer 150 can optionally include subtractivecolorants as discussed earlier. The panchromatic scattering porousparticles 125 in the underlying layer 150 can optionally be replaced bya mixture of porous particles 125 a, 125 b, 125 c, each having a singlepore size as was discussed relative to FIG. 4B. FIG. 8D illustratesanother embodiment that combines the low-specular reflectance coating120 of FIG. 8B with the underlying layer 150 of FIG. 8C.

Fabrication of Porous Particles

Porous Particle P1 (used in Inventive Examples 1A and 1B describedbelow) was made using the WOW PLC method as taught in U.S. Pat. No.8,703,834 using a monomer oil phase O comprising 90% methyl methacrylate(MM) monomer and 10% CN301, a polybutadiene dimethacrylate oligomer(Bdma) from Sartomer Americas (a part of Arkema Americas). To this, 1.9%Grindsted® PGPR 90 from Dupont de Nemours, Inc., was added as anemulsion stabilizer. The porogen first water phase W1 comprising 1.95%water solution Aqualon™ CMC-9M31F from Ashland was prepared and used inW1-to-O weight ratio of 2-to-3. The porogen water phase W1 was mixedinto oil phase O and then emulsified at high energy to form a firstemulsion W1/O. An initiator was incorporated at 0.8% of the monomerweight by adding a 25% solution of Vazo™ 52 from The Chemours CompanyFC, LLC in ethyl aetate to the first emulsion W1/O. A second water phaseW2 was prepared using 5% solution Ludox® TM from W. R. Grace and Companyin a 207 mM pH 4 citrate buffer resulting in a W2 with 2.5% colloidalsilica. The W1/O was mixed with W2 in an ice bath at a ratio of 3 partsW1/O to 5 parts W2. The mixture was then emulsified at a lower energy tocreate a stable double emulsion. The double emulsion was then added toequal parts water heated to 55° C. and held overnight to form poly MMco-Bdma (CN301). The resulting porous particles had a 6.8 micron numbermode as measured by a Sysmex FPIA2000 with 23% porosity measured by Hgintrusion in the form of isolated spherical macro pores.

The Porous Particle P2 (used in Inventive Examples 1B and 1C describedbelow) was prepared using the same method as for Porous Particle P1except that a mill dispersion of carbon was added to W1 to provide 0.2weight percent carbon based on the dry weight of the porous particle.The carbon was modified as described in U.S. Pat. No. 5,851,280, whichis incorporated herein by reference, using a freshly prepared diazomiumsalt made by reacting 3 parts 10% 4-ethylaniline in water with 1 partconcentrated hydrochloric acid below 12° C. for 30 minutes, adding 0.6%urea and stirring for 5 minutes then filtering. The diazonium saltsolution was adding at 1 part to 10 parts 14% Regal 330 carbon blackdispersion in water and stirred for 3 hours then filter, rinsed, andvacuum dried at 50° C. overnight. The modified carbon was then milled to100 nm volume mode carbon dispersion.

Porous Particle P3 (used in Inventive Example 1D described below) wasprepared using the WOW ELC as described in U.S. Pat. No. 7,754,409comprising Kao N, which is a polyester resin obtained from KaoSpecialties Americas LLC, a part of Kao Corporation, Japan, dissolved inethyl acetate, 0.2% Efka® 6225 from BASF as an emulsion stabilizer, and1% carbon in the oil phase of the WOW. The porogen first water phase W1comprising 1.95% water solution 250,000 Mw CMC from Acros OrganicsB.V.B.A. of New Jersey, part of Thermo Fisher Scientific, was preparedand used in W1-to-O weight ratio of 2-to-3 and emulsified. A secondwater phase W2 was prepared using 3.6% solution Nalco™ 1060, and 50 nmcolloidal silica dispersion from Nalco Chemical Co. of Chicago, Ill., ina pH 4 citrate/phosphate buffer resulting in a silica concentration of1.8% colloidal silica dispersion. The W1/O was mixed with W2 at a ratioof 3 parts W1/O to 5 parts W2 emulsified to create a stable doubleemulsion, diluted 1:1 with water, then passed through a flash evaporatorto remove the solvent in a continuous pilot process. Porous particle P3had a number mode size of 4.9 microns was 31.6% porous.

Porous Particles P4 and P5 (used in Inventive Examples 2A and 2Bdescribed below) were made in a similar manner to Porous Particle P1except that a solvent dye was dissolved in the monomer oil phase O.Porous Particle P4 contained 0.2% Orasol® Blue GL, a C. I. Solvent Blue70 from Kremer Pigmente GmbH (now sold as Orasol Blue 855 by BASF).Porous Particle P5 contained 0.2% Valifast® Orange 3210, a mixture ofsolvent dyes from Orient Chemical Industries. Porous Particle P4 had 6.8micron number modal size and 21.7% porosity while Porous Particle P5 hada 6.3 micron number modal size and 24% porosity.

Porous Particles P6, P7, and P8 (used in Inventive Examples 3 and 4described below) were made using the DOW PLC method to produce 3different sized particles having impermeable shells includingcross-linked poly MM co-Hdda. An oil phase including equal parts MM andHdda monomers were mixed in a ratio of 3-to-2 with a porogen phasecomprising 4 parts cyclohexane to 1 part ethyl acetate. To this wasadded an initiator Vazo™ 52 from The Chemours Company FC, LLC at 0.85%by weight of monomer. Ibis oil phase was added in a ratio of 3-to-5 to awater phase comprising water, a stabilizer (Ludox® TM 20 nm colloidalsilica from W. R. Grace and Company), a promoter (methyl aminoethanoladipate), and a second stabilizer (poly(2-ethyl-2-oxazoline) availableas Aquazol® 50 by Polymer Chemistry Innovations, Inc.), with the levelof the colloidal stabilizer and promoter varied to obtain differentparticle sizes. The mix was emulsified and reacted 45° C. overnight thenfinished by solvent extraction as described above. Porous Particles P6,P7, and P8 were made using 40% porogen comprising 20% ethyl acetate and80% cyclohexane and were made at 3 particle sizes of 5.4, 12, and 30micron mean surface weighted diameter, respectively, as measured by aCoulter Counter and a porosity of about 33% using the Coulter Counterand Aerosizer method. The pores comprised isolated meso pores of 15 to50 nm and networked macro pores less than 200 nm in size with a Miebackward scattering effective size of 60 nm.

The Porous Particle P9 was made using the DOW PLC method described forPorous Particles P6 comprising MM at 50% and 50% Tmpta using 40% porogencomprising 5% ethyl acetate and 95% cyclohexane was made at a particlesize of 4.8 micron number mode diameter as measured by a Coulter Counterand a porosity of about 35%. The pores comprised networked micro andpores of about 50 nm in size and macro pores from 100 to 1500 nm insize.

Porous Particle P10 comprising 50% MA and 50% Tmpta was made using theDOW PLC method described for Porous Particles P9 using 35% porogencomprising 5% ethyl acetate and 95% cyclohexane was made at a particlesize of 18.1 micron number mode diameter as measured by a CoulterCounter and a porosity of about 43%. The resulting Porous Particle P10had a 10 nm impermeable polymer shell surrounding interconnected macropores about 100 to 2000 nm is size. ICP measurement of the porousparticle found most of the colloidal silica present in the wet cake ofthe porous particle but was mostly lost during the cryo-face-offsectioning described Evaluation Methods below as seen in FIG. 2A.

Porous Particle P11 included as a comparative example was made with 40%styrene and 60% divinyl benzene using the DOW PLC method and a diluentcomprised 25% cyclohexanol and 75% toluene. The log Po/w and solubilityin water of styrene are 2.7 and 0.03% respectively while those ofdivinylbenzene at 3.8 and 0.005%. As expected, the particle had nopolymer shell and an inorganic shell was not supported resulting infragile particles with an open porosity. The particles were about 12microns in size but could not be measured by typical techniques such aselectrical sensing zone methods (e. g. Coulter Counter), sedimentation,sieving, and light scattering due to the open porous structure,fragility, and small size.

Table 1 shows a comparison of the Porous Particles P1-P11, summarizingthe corresponding fabrication method and particle composition.

TABLE 1 Porous particles used in example coatings Par- ticle MethodPolymer Porogen D_(n,mode) Porosity Pigments P1 WOW 90/10 MM/Bdma 2:3W:O 6.8 23.0% None PLC P2 WOW 90/10 MM/Bdma 2:3 W:O 6.8 21.7% 0.2% PLCCarbon P3 WOW Kao N 2:3 W:O 4.9 31.6% 1.0% ELC Carbon P4 WOW 90/10MM/Bdma 2:3 W:O 6.8 217% 0.2% PLC Orasol Blue GL P5 WOW 90/10 MM/Bdma2:3 W:O 6.8 21.7% 0.2% PLC Valifast Orange 3210 P6 DOW 50/50 MM/Hdda 2:3D:O 4.9 33.1% None PLC P7 DOW 50/50 MM/Hdda 2:3 D:O 11.4 35.1% None PLCP8 DOW 50/50 MM/Hdda 2:3 D:O 24.5 33.2% None PLC P9 DOW 50/50 MM/Tmpta2:3 D:O 4.8   35% None PLC P10 DOW 50/50 MA/Tmpta 1:2 D:O 18.4   44%None PLC P11 DOW 40/60 Styrene co 3:2 D:O N/A N/A None PLC divinylbenzene

Coating Formulations and Methods

The various layer formulations used in the practice of this inventioncomprise an aqueous dispersion of the desired components. For example,the multispectral camouflage coating formulation typically includeporous particles as an additive colorant, and optionally include amatrix polymer and subtractive colorants, all mixed together in water toform a stable aqueous dispersion. The multispectral camouflage coatingformulations generally have a solids content of 30-40% while lowspecular reflectance overcoat formulations have a solids content of15-25%.

In some embodiments, the matrix polymer can be chemically crosslinked.The coating formulations can optionally include relatively smalleramounts of other materials, such as crosslinking agents, tintingcolorants, thickeners, and pH control agents.

For lower solids content, thickeners can be included to enhance theformulation viscosity if desired. Known thickeners can also be utilizedto control the rheology of the multispectral camouflage coatingformulation depending upon the method used to apply it to a substrate(or underlying layer). Particularly useful rheology modifiers areRheovis® PU 1214 (BASF), Acrysol® G111 (Dow Chemical Company), andCarbopol® Aqua SF-1 (Lubrizol).

Agents capable of being crosslinked under appropriate conditions aftercoating may be added to provide improved insolubility of a particularlayer in water and promote the adhesion to the substrate or optionalunderlying layers. The crosslinking agent is a chemical havingfunctional groups that are activated chemically with heat, radiation, orother means that are capable of reacting with reactive sites on thelatex polymer under curing conditions to thereby produce a crosslinkedstructure. Examples of suitable crosslinking agents includemulti-functional aziridines, aldehydes, and epoxides.

Drying and optional crosslinking of the matrix polymer in the layerformulation can be accomplished by suitable means such as by heating,and various mechanisms can be employed for crosslinking the matrixpolymer. For example, the crosslinking can involve condensation oraddition reactions promoted by heat or radiation. In one embodiment, alatex composition is used as the matrix polymer. Upon heating, the latexfilm dries, with a crosslinking reaction taking place between thereactive side groups of the polymer chains. Such a latex is referred toas a thermoset emulsion. If the particular latex polymer used is notitself heat reactive, then suitable catalysts or crosslinking agents canbe added to promote crosslinking upon heating.

One skilled in the art would understand that other coating vehicles andfilm forming can be used, including high vapor pressure organic solventseither alone or in combination with water to obtain the desiredformulation quality.

The multispectral colorant and underlying layer formulations can bethusly prepared and coated or otherwise applied onto a substrate by anyof a number of well-known techniques, such as wrapped wire rod coating,blade coating, spray coating, air knife coating, gravure coating,reverse roll coating, slot coating, extrusion hopper coating, slidecoating, curtain coating, spray coating, foam coating, froth coating,rotary screen coating, pad coating, and other techniques that would bereadily apparent to one skilled in the art.

After application of the multispectral camouflage coating formulation(and underlying layer formulation if used) to the substrate, eachformulation is generally dried by simple evaporation of water (and anyother solvents) from the applied formulation and from the pores of theporous particles, which drying can be accelerated by known techniquessuch as convection heating to provide an article of the presentinvention.

The resulting articles can have any desired overall average drythickness, but in most embodiments, the overall average dry thickness isat least 15 μm or typically at least 100 μm. This overall averagethickness includes any dry substrate thickness described above(including any subbing or adhesion layers) and any overcoat (includingthe low specular reflectance coatings described above) as well as anaverage dry thickness of the multispectral camouflage coating of atleast 100 μm. All of these “average” dry thicknesses are estimated fromthe coating formulation and the wet coverage.

Evaluation Methods:

Freeze fracture samples were prepared by cooling the sample with liquidnitrogen and impacting with a sharp instrument such as a razor blade.Cryo-face-off sections were prepared by infusing wet samples withsucrose for more than one week, freezing in liquid nitrogen, sectioningin a microtome while frozen but surrounded by a liquid, and collectingthe floating section on a transmission electron microscope (TEM) grid.Freeze fracture and cryo-face-off samples supported by the TEM grid wereimaged using a Hitachi S-4100 SEM.

Total diffuse reflectance of normal illumination spectra was measuredwith Varian Cary 5E UV-VIS-NIR Spectrophotometer using and integratingsphere to collect the total diffuse reflectance.

Aerial imaging was performed with paired Nikon D5500 DSRL cameras withNikon 105 mm F4.5 UV lenses where one of the cameras was customized bynot having the RGB filter coating applied to the sensor and one havingeither a Hoya R72 lens filter for NIR capture or a Baader U-Filter UVBandpass filter for UV capture while the other camera was used for VIScapture. A contrast was calculated as the minimum of the absolutedifference between average reflected intensity for the target in eachwavelength band and either the maximum or minimum reflected intensity inthe frame excluding the target and reference at near range is used toexpress the detectability against the natural background at long rangedistance where the target may fill only a few pixels of the digitalcamera. The same contrast calculation using only the sky lit or shadowedportion of the target to express the identifiability of the target whenimaged at a mid-range distance.

The specular reflectance viewed from a distance create glints that makean object detectable. Glare arises from off angle specular reflectancedue to imperfections. Glint and glare were evaluated as the contrastadded to the detectability. The ultimate added contrast at close-rangevaried with wavelength and was not always definable due to thesaturation of the camera sensors for a given color, specularreflectivity, and camera exposure setting. The glint of a coating may berepresented as the number of pixels expressed as the diameter forspherical and cylindrical targets where the contrast for a single pixelimaging the glint is increased by 0.5 taking into account themagnification effects of curved surfaces the image of the sun and theincluded angle at the target surface between the target and camera. Thissize in pixels was relatively invariant with wavelength and can be usedto estimate the distance that a target may be detected given the imagingsystem MTF and magnification. For the low specular reflectance coatings,it was found that the ultimate added contrast was much lower than 0.50and the pixels size to achieve this added contrast does not exist.

Coating toughness was evaluated by a micro-scratch test load at whichcoating damage other than compression occurred using Microscratch Testerdeveloped and produced by CSM Instruments, Switzerland. All samples wereconditioned for at least 24 hours at 70F/50% RH prior to testing. Afterthis conditioning period, 3 ramped load scratches were generated in the(3.06-550 g) load range on each sample using a 75 micron radius, 90degree conical diamond stylus as the abrader. A scratch velocity of 5mm/min, a scratch length of 5 mm, and a loading rate of 546.94 g/minwere used in all cases. An attached optical microscope was used toobserve scratch track morphologies and determine the load required toinitiate damage to the surface of each sample. All samples tested werebacked by vinyl designed to be compliant and conform to shaped surfaceswhen applied to the target surface and therefore had plastic deformationdue to compression at the lowest loads of 3 g.

Evaluation of coating impermeability were conducted by placing a drop ofwater on the coating and observing the loss of light scattering leadingto a darker color due to the increased transmittance to the blacksubstrate. Onset times were recorded when the drop was be spread andthere was a just noticeable difference between the initial and newlywetted areas in diffuse light. Completion was recorded as the time whenthere was no further loss of light scattering. Water in the drop lost toevaporation was replenished periodically for longer observation times.

Comparative Example 1: Leaves

The total diffuse reflectance values of a single leaf over black for thefront and back for maple, oak, and plum leaves are given in Table 2. NIRreflectance values approach 80% or higher for multiple leaves due to thelack of absorption in the NIR. SWIR reflectance values are lower thanNIR due to absorption by water and hydroxides in the SWIR and multipleleaves may not achieve as high of a total reflectance as that seen inNIR. Low UV reflectance values are due to the strong absorption of UV bychlorophyll in the leaves.

TABLE 2 Measured Leaf characteristics. % Reflectance % Illuminated inAmbient Light Reflectance UVB UVA VIS NIR SWIR IR IR IR 280- 320- 400-720- 1000- LED LED LED 315 380 700 1000 1800 760 880 920 nm nm nm nm nmnm nm nm Maple Front 5.4 5.1 5.5 41.7 32.9 42.7 43.6 45.0 Maple Back 5.14.9 8.3 36.4 29.3 36.8 37.5 39.0 Oak Front 4.4 4.1 5.1 44.8 34.8 45.646.6 47.3 Oak back 5.7 5.2 11.6 43.5 35.3 44.2 44.2 45.1 Plum Front 5.04.8 6.4 49.1 39.4 48.9 51.7 51.0 Plum Back 4.7 5.0 9.0 47.5 39.4 47.148.9 49.4

Comparative Example 2: Woodland Pattern Camouflage Fabric

The Woodland Pattern camouflage fabric is a commercially availablefabric developed in 1948 by ERL and put into use in Viet Nam thenregularly in 1980's. The Woodland Pattern has some NIR contrast betweenblack and other colors but no high NIR reflectance color to emulategreen foliage. The green NIR reflectance is lower than the tan and brownrisking exposure of camouflaged object or operator by hyperspectralanalysis. Measurements for the colors used in the Woodland Pattern areshown in Table 3.

TABLE 3 Measured Woodland Pattern camouflage fabric characteristics.Reflectance Illuminated in Ambient Light % Reflectance % UVB UVA VIS NIRSWIR IR IR IR 280- 320- 400- 720- 1000- LED LED LED 315 380 700 10001800 760 880 920 nm nm nm nm nm nm nm nm Woodland 12.8 14.2 19.0 31.834.0 29.9 32.5 33.6 Tan Woodland 12.0 13.3 17.9 29.5 31.7 27.7 29.9 31.1Brown Woodland 8.6 9.6 13.9 22.4 24.8 21.8 22.0 23.5 Green Woodland 7.47.9 9.8 16.4 19.5 15.1 16.5 18.0 Black

Comparative Example 3: Marine Pattern Woodland Camouflage Fabric

The Marine Pattern (MARPAT) Woodland camouflage fabric as described inU.S. Pat. No. 6,805,957 contains four colors (brown, green, khaki andblack). Total diffuse reflectance measurements for three of the colorsare shown in Table 4. The black color was not present in large enoughareas to measure on the sample. Black is carbon base and provides highcontrast. There is no mid-level NIR reflectance resulting in a starkblack and with NIR image with mostly high NIR reflectance.

TABLE 4 Measured Marine Pattern Woodland camouflage fabriccharacteristics. Illuminated Reflectance in Ambient Light % Reflectance% UVB UVA VIS NIR SWIR IR IR IR 280- 320- 400- 720- 1000- LED LED LED315 380 700 1000 1800 760 880 920 nm nm nm nm nm nm nm nm MARPAT N/A N/A12.4 52.4 57.7 36.7 59.6 61.9 Brown MARPAT N/A N/A 8.3 49.8 58.4 27.659.6 61.9 Green MARPAT N/A N/A 15.8 56.0 59.4 41.9 62.9 63.8 Khaki

Comparative Example 4: MultiCam Camouflage Fabric

The MultiCam camouflage fabric, based upon U.S. Pat. No. 9,062,938, ismade using skewed and oscillating print rollers for a second colorprinted over other colors providing a wide range of colors. Totaldiffuse reflectance measurements for some selected areas are shown inTable 5. There was no low NIR reflectance color in the sample. Thedarkest color was perceived as a brown and did not contain carbon. Greenhad lower NIR reflectance in the very near infrared (vNIR). Overall,there is a good range of contrast for total light amplification nightvision devices, but there is a risk of exposure when using NIR onlynight vision devices or hyperspectral analysis.

TABLE 5 Measured MultiCam camouflage fabric characteristics. ReflectanceIlluminated in Ambient Light % Reflectance % UVB UVA VIS NIR SWIR IR IRIR 280- 320- 400- 720- 1000- LED LED LED 315 380 700 1000 1800 760 880920 nm nm nm nm nm nm nm nm Brown 3.9 4.8 9.1 36.8 54.0 23.7 47.4 51.8Light Brown 3.7 4.3 14.1 53.2 59.4 46.2 59.4 61.1 Tan w/Green 7.0 9.628.2 54.8 58.5 50.3 58.4 59.7 Green 3.9 4.8 13.5 31.9 54.2 24.8 35.440.7 Light Green 4.1 5.0 18.5 37.5 59.0 29.9 39.7 46.9 Gray Green 5.37.1 22.1 42.0 56.5 34.9 45.8 49.9 Silver Gray 12.1 17.7 40.9 59.9 59.956.9 62.9 62.6

Comparative Example 5: Rust-Oleum Paints

The matte paints in typical camouflage colors are provided by Rust-oleumcompany. The total diffuse reflectance values are given in Table 6 for aselection of these paints spray coated on different colored substrates.There is relatively small range of reflectance in the IR and littlereflectance in UV. These matte paints were found to have 4 to 10 timesthe specular reflectance for light incident at 85 degrees from thenormal compared to the low specular reflectance coatings described incommonly-assigned U.S. patent application Publications 2020/0199373,2020/0199379 and 2020/0199381 to Lofftus.

The NIR/SWIR reflectance for khaki is reasonable for desert terrain butthick coatings are required to hide the substrate especially in SWIRwhere the opacity is low for Khaki. All the green paints had NIR/SWIRreflectance values less than khaki while natural greens of vegetationhave reflectance values greater than 70 percent. While some contrast maybe achieved between khaki and the different green paints, the overallreflectance in NIR/SWIR is too low increasing the detectability of thetarget. Hyperspectral analysis would reveal that the green VIS does notmatch the high reflectance in NIR/SWIR expected from vegetation furtherenabling detection.

The low and constant reflectance in UVA/UVB of the green paints isconsistent with strong absorption of UV by chlorophyll in green plants.However, specular reflectance of UV from small surfaces such as grassblades and leaves increase the UV contrast in a natural scene. Higher UVreflectance values are seen for dry grass, bark, sand, and stone thatkhaki paint fails to provide.

TABLE 6 Measured Rust-oleum paint characteristics. % Reflectance %Illuminated in Ambient Light Reflectance UVB UVA VIS NIR SWIR IR IR IR280- 320- 400- 720- 1000- LED LED LED 315 380 700 1000 1800 760 880 920nm nm nm nm nm nm nm nm 279176 6.2 6.6 18.2 17.9 14.1 18.3 17.9 17.6Army Green 279177 6.8 7.4 25.2 26.9 22.8 28.3 25.9 25.5 Khaki on AlKhaki on 7.5 8.0 24.0 23.4 15.6 25.2 22.2 21.8 black Khaki on 7.5 8.024.6 27.9 30.1 28.6 27.1 27.9 white Lt Green on 6.5 6.8 22.2 23.2 17.723.5 23.5 23.0 black Lt Green on 6.5 6.8 22.0 23.0 18.3 23.3 23.2 23.1white 279176 Deep 5.6 5.6 9.8 8.8 7.0 9.2 8.4 8.5 Forest Green on black279176 Deep 5.6 5.6 9.7 8.8 7.0 9.1 8.5 8.5 Forest Green on white

Comparative Example 6: Mylar Balloon

An inflated Orbz spherical party balloon consisting of a sphericalmetalized PET (Mylar) with white and black coating in the pattern of asoccer ball was imaging in conjunction with the inventive examples as areference. The white area of this comparative example consisted of anadditive colorant that was subtractive in UVA/B but thin enough to allowmoderate UVA/B reflection from the metal coating. The combinedscattering and absorbance in the UV by the white colorant reduced the UVspecular reflectance from the metal coating without significantlyreducing the VIS specular reflectance. The black areas of thiscomparative example consisted of a subtractive colorant for all theimaged wavelength bands (UVA/VIS/NIR). The ultimate added specularreflectance of glint and glare for the black was about 0.90, 0.89, 0.87,and 0.62 for red, green blue and UV respectively at an included anglebetween the sun, target surface, and observer of 90 degrees. No imageallowing analysis of glint and glare on the black reference was capturedin IR. The number of pixels for an added contrast of 0.50 was 14.3pixels and 10.7 pixels for spheres and cylinders respectively.

Glint on black reference saturates an area of the array camera at lowexposure that is about twice the diameter the image of the sun. Glare onblack reference at low exposure increases reflected intensity in acontinuous manner from the black with no glare at 0.1 reflectedintensity for black to 1 at the glint. The glare adds 0.5 to thedetection signal for a target having the specular properties of thereference when the target is cylindrical and 11 pixels in diameter orfor a spherical that is 14 pixels in diameter. The detectability of atarget is not strongly influenced by the glint and glare when the targetfits completely within a single pixel of the imaging system. The effectof glint and glare on detection becomes significant at closer positionswhere the target fills more than 10 pixels.

Glint and glare on the white reference saturates camera even at lowexposure settings with the saturated area being much greater than theimage of the sun in VIS and NIR. Due to the absorbance of UV by thewhite pigment in the white reference, the UV glint increased thereflected intensity by 0.3 and the glare by 0.15 above that of thediffuse UV reflectance from the white reference.

Inventive Example 1: Nighttime Multi-Spectral Camouflage Fabric

It is desirable for nighttime camouflage to be black in VIS with absentof visible light sources but have contrasting reflectance in NIR toprovide detection against NIR security light sources. The combination ofprocess black using colorants that are non-absorbing in the NIR (forexample a combination of yellow, cyan, orange, and magenta pigments togive a VIS black) with porous polymer particles having macro pores inthe micron size range provide a visible black with high reflectance inthe NIR and SWIR. High contrast in NIR is obtained while maintaining VISblack by varying the amount of carbon black in the coating. InventiveExample 1A-1D represent four coatings on a white cotton cloth where thecoatings contain the same level of VIS colorant that are non-absorbingin NIR while varying the level of the broadly absorbing carbon.

Four fluids of different compositions suitable for screen printing wereprinted in adjacent areas of the white cotton cloth Sew Classic BTTMWGHTwhite Wrinklease 7 oz. per yard fabric by rolling viscous coating fluidthrough multiple filament nylon thread woven into mesh where the threadswere spaced at 360 μm resulting in 200 μm openings for a 31% open area.These fluids were designed to give a consistent black in VIS whilehaving a high variation in NIR reflectance. Three Porous Particles P1,P2, and P3 were prepared for use in the four fluids with Porous ParticleP1 used in Inventive Example 1A, equal parts Porous Particles P1 and P2used in Inventive Example 1B, Porous Particle P2 used in InventiveExample 1C, and Porous Particle P3 used in Inventive Example 1D.

The four coating fluids were prepared by combining the particles at avolume ratio of 2-to-1 matrix polymer with an acrylic latex NeocrylA-6093 from DSM, adding a viscosity modifier Rheovis PU1214 NC fromBASF, and adding a combination of milled pigment water dispersions of PY155, PO 35, PR 185, and BP 15:3. All of the fluids were prepared at38-40 volume % solids plus pores contained within the porous particles.Inventive Example 1A corresponds to the configuration in FIG. 6A wherethe subtractive colorant is distributed within the binder, whileInventive Examples 1B, 1C, and 1D are a combination of theconfigurations of FIG. 6A and FIG. 6B so that the subtractive colorantis distributed both within the binder and inside of the pores.

The coating fluids were screen printed on the fabric using a PET stencilto create a different patterned image for each fluid. The coating wasdried between each coating each fluid and the final coated cloth withthe camouflage pattern cured at 125° C. for 20 minutes. These coatedfabrics demonstrate the use of porous particles with pore sizes thatscatter VIS and IR wavelengths, visible colorants, and a broadlyabsorbing neutral colorant to achieve a high variation of NIRreflectance while maintaining a consistent VIS black enabling blackcamouflage with a good match in NIR for natural backgrounds includingvegetation, soils, rocks, and shadows. The NIR reflectance for InventiveExample 1A was greater than the cloth and near to that observed formultiple leaves while having a low reflectance in VIS.

Table 7A summarizes the formulations for Inventive Examples 1A-1D, andTable 7B summarizes the corresponding measured reflectancecharacteristics.

TABLE 7A Formulations for Inventive Examples 1A-1D. Ex- AdditiveColorant % Subtractive Colorant ample Particle % Carbon PY 155 PO 38 PR122 PB 15:3 1A P1 59 0.00 0.54 1.45 0.43 1.06 1B P1 & P2 59 0.06 0.551.50 0.43 1.06 1C P2 59 0.12 0.57 1.55 0.43 1.06 1D P3 54 0.54 0.58 1.810.48 1.12

TABLE 7B Measured reflectance characteristics for Inventive Examples1A-1D. % Reflectance in Ambient Light % Illuminated VIS NIR SWIRReflectance 400- 720- 1000- IR LED IR LED IR LED Example 700 nm 1000 nm1800 nm 760 nm 880 nm 920 nm Cloth 65.9 67.2 60.9 67.0 67.6 67.1 1A 8.663.5 70.2 30.0 76.6 75.7 1B 8.3 44.9 53.3 25.7 52.5 52.7 1C 7.3 25.032.1 18.3 27.7 28.7 1D 10.2 15.7 17.1 15.5 15.7 16.8

Inventive Example 2: Daytime Multi-Spectral Camouflage Fabric

At times it is not possible to achieve the desired VIS color with abroadly absorbing neutral colorant such as carbon. Inventive Example 2A,corresponding to FIG. 6A, demonstrates the using of porous particles andVIS colorants without the use of a broadly absorbing colorant to achievea dark green that is highly reflective in NIR and SWIR and emulateleaves. Inventive Example 2B, corresponding to a combination of FIG. 6Aand FIG. 6B, demonstrates the use of porous particles with pore sizesthat scatter VIS and IR wavelengths to mask metallic luster and glintsfrom colored metallic pigment is moderately absorbing in NIR and SWIR toachieve tans with a mid-level reflectance in NIR and SWIR to emulatesoils, sand, and rock commonly found in desert environments. Thesesamples were fabricated using a method similar to that described abovefor Inventive Example 1.

Table 8A summarizes the formulations for Inventive Examples 2A-2B, andTable 8B summarizes the corresponding measured reflectancecharacteristics.

TABLE 8A Formulations for Inventive Examples 1A-2B. Additive %Subtractive Colorant Colorant SO PY PO PR PB Part. % Carbon SB 70 MixBrass 155 38 185 15:3 2A P4 43 0.00 0.09 0.00 0.00 0.54 0.06 0.00 0.082B P2/P5 52 0.03 0.00 0.08 1.17 0.18 0.00 0.33 0.02

TABLE 8B Measured reflectance characteristics for Inventive Examples2A-2B. % Reflectance in Ambient Light % Illuminated Reflectance Ex- VIS400- NIR 720- SWIR 1000- IR LED IR LED IR LED ample 700 nm 1000 nm 1800nm 760 nm 880 nm 920 nm Cloth 70.3 71.1 64.4 71.1 71.3 71.1 2A 14.7 29.933.0 28.9 30.4 31.6 2B 9.6 61.0 63.8 35.7 72.2 72.2

Aerial Camouflage Coatings

Lighting conditions must be considered when designing colorants formultispectral camouflage. The relevant light source during the daytimeis sunlight after atmospheric absorption and scattering. Atmosphericscattering of the solar spectrum absent clouds and dust is greatest forthe UV bands and least in the IR and SWIR bands. The scattering oflonger wavelengths increases as atmospheric moisture droplets and dustincrease in number and size.

The daylight illumination of an target is highly diffuse in UVA/B fornearly all directions above the target. The illumination at longerwavelengths may be specular for clear conditions or diffuse for cloudyconditions. The underside of airborne targets at very high altitudeswill be illuminated by the scattered sunlight. For airborne targetsclose to the ground, the underside will be lit by sunlight reflectionfrom the ground. Sandy and stony ground will reflect most wavelengths ata moderate level with the least being reflected for the UVA/B range.Green vegetation is highly reflective in the NIR and SWIR, slightlyreflecting in the green range of VIS, and non-reflective in the UVA/B aswell as the blue and red ranges of VIS. Snow scatters light like theatmosphere for cloudy conditions and is highly reflective in allwavelengths for UVB to NIR and strongly reflective in SWIR.

An effective camouflage design for airborne targets is to emulateatmospheric scattering against the blackness of space. The underlyinglayer should have a broad band subtractive colorant to emulate theblackness of space. Over this, an atmospheric scattering emulation layercomprising porous polymer particles with pores sizes to emulate variousbackgrounds against which the target is viewed such as clear or cloudyconditions. A final low spectral reflectance topcoat is applied toreduce or eliminate spectral reflections.

The atmospheric scattering emulation for clear conditions may contain ablue colorant to reduce the effect of long light paths through thecoating at oblique incidence and viewing angles. A viewing angle that isoblique to the surface collects light over a larger portion of thesurface and the coating will be more reflective than at a viewing anglenormal to the surface. For example, the light scattered to an observerby the atmospheric emulation coating from the image edge of the circularfuselage emulates the atmospheric scattering at the horizon and appearstoo reflective when the aircraft is viewed against the darker blue of aclear sky at high elevation angles. A majority of light passingobliquely to the coating surface will pass through more of the coatingand scattering from the layer increases the reflectance. Thisreflectance can be reduced by adding low levels of a subtractivecolorant such as in Inventive Example 3 (350 ppm Direct Blue 71 coatingas shown in FIG. 7). In this case, the amount of subtractive coloranthas very little effect on the reflectance spectrum of normal incidencelight. Higher levels of blue colorant can be used to sharpen the spectraand provide a greater UVA/B reflectance while maintaining a lowreflectance in green, provided that the colorant has a low level of UVabsorbance, such as in Inventive Example 4 (1760 ppm Direct Blue 71coating as shown in FIG. 7).

An atmospheric scattering emulation layer that works for aerial targetsin cloudy conditions such as those used in Inventive Example 5 alsoworks well to emulate snow for ground-based targets. The reflectance ofthe underlying layer can be adjusted to better match the reflectance ofthe ground beneath the snow. These UVA/B reflective coatings may beinterspersed with UVA/B absorbing and color regions to match non-snowcover objects in the natural scene.

Inventive Example 3: Aerial Blue-Sky Camouflage Coating

A first coating fluid for use as a scattering layer to emulate blue sky,corresponding to FIG. 6A, was made using the 5.4 micron particle P6 at43 volume % solids plus pores and at volume ratio of 2 parts by volumeparticles to 3 parts matrix polymer, where the matrix polymer comprised99% Hycar® 26120 (a thermoset acrylic emulsion from Lubrizol with aT_(g) of −11° C.) and 1% Carbopol® Aqua SF-1 (a viscosity modifiercomprising a lightly cross-linked acrylate copolymer from Lubrizol). Asolution of direct blue 71 was added to obtain a level of 353 ppm in thefinal dry coating. The Hycar® 26120 provided good flexibility to allowstretching and formability of the laminate when applied over curvedsurfaces.

A second coating fluid to act as a low specular reflectance overcoat,corresponding to FIG. 8B, was prepared using the 5.4, 12, and 30 micronPorous Particles P6, P7, and P8 in a ratio of 4-to-2-to-1 at 22 volume %solids and at a volume ratio of 3 parts by volume particles to 2 partsmatrix polymer, where the matrix polymer comprised 12% Hycar® 26120, 83%Carboset® CR675 (a thermoplastic styrene-acrylic copolymer emulsion fromLubrizol having a minimum film forming temperature of 34° C. and a T_(g)of 70° C.) and 1% Carbopol® Aqua SF-1. The level of Hycar® 26120Carboset® CR675 were chosen to balance scratch resistance andflexibility.

The first coating fluid was coated on corona discharge treated (CDT) 3M™Wrap Film Series 1080 matte black vinyl using a 3 mil gap hand coatingblade resulting in blue sky emulation layer of about 31 gsm and about 30microns in thickness once dried. The same fluid was then coated in acontinuous roll coating process using a slot die resulting in a driedcoating of 37 gsm and a thickness of about 35 microns. The second fluidwas coated by the same methods resulting in a low reflectance overcoatat 15 gsm that was also emulated blue sky with an average thickness ofabout 15 microns but with hemispherical features up to 25 microns abovethe first layer resulting in an coated article corresponding to FIG. 8D.The coatings were dried and cured to crosslink the Hycar® 2610. Totaldiffuse reflectance was measured on the samples of the blade and slotcoated films. The measured reflectance values (shown in Table 9A)decrease monotonically with light wavelength providing no additionalreflectance above that observed from the vinyl substrate in SWIR.

TABLE 9A Measured reflectance characteristics for Inventive Example 3. %Total Diffuse Reflectance of Normal Incident Light UVB UVA VIS-B VIS-GVIS-R NIR SWIR 280- 320- 400- 500- 600- 720- 1000- 315 380 500 600 7001000 1800 Sample nm nm nm nm nm nm nm 3M Vinyl 5.5 5.1 5.0 5.1 5.0 4.84.6 Blade Coating 29.8 28.9 17.8 10.8 8.0 6.3 4.7 Slot Coating 30.7 33.222.0 12.5 8.9 6.7 4.5

The two coating fluids were then diluted 5 parts fluid to 1 part waterand spray coated on an inflated spherical 16 in black Orbz balloon thathad been painted with Rust-oleum Universal 245197 satin black to achievesufficient absorbance in the IR. The balloon was imaged in UV, VIS, andNIR spectral ranges against a blue sky about 50 to 75 feet above a greengrass field in late spring.

Table 9B summarizes various detectability and identifiability metricsevaluated from the imaged balloons with the Inventive Example 3 coating,as compared to white and black reference samples (Comparative Example6). As described above in the Evaluation Methods section, the ability todetect an target at a distance is the greatest value of either thedifference between the average reflectance of the lit portion of thetarget minus the reflectance of the brightest region of the localbackground, the sky in this case, or the darkest region of the localbackground minus the average of the shadowed portion of the target. Thismetric applies at a distance where the area of any portion of the targetis a few pixels in the imaging system and can be distinguished from thebackground. The number of pixels filled by the portion of the targetneeded to detect the target for an imaging system with a given MTF andmagnification increases with a decreasing detectability and requires thetarget to be closer for it to be detected. Inventive Example 3 exhibitsa significant reduction in the detectability of the target in UV, blue(B), and NIR spectral ranges while being virtually undetectable in thegreen (G) and red (R) ranges. The shadowed portion of the balloonexperienced a lack of lighting in UV and a strong lighting in NIR fromsun light reflected off the grass resulting in an increaseddetectability in those spectral ranges. At much higher elevations, mostof the lighting of the shadowed portion of the target would beatmospheric light scattered from the horizon resulting in a better matchto the lit portion of the target and a decreased detectability.

The ability to identify a target at a distance relies upon having enoughpixels of the outline of the target to distinguish its unique profile.The target becomes unidentifiable at a distance if the reflectance ofeither the lit or shadowed portion of the target falls within the rangeof the local background. The identifiability metric determines thedistance at which all portions of the target can be distinguished fromthe background for a given imaging system MTF and magnification.Inventive Example 3 exhibits a significant reduction in theidentifiability of the target in UV and NIR spectral ranges while beingvirtually unidentifiable in the VIS (B, G, R). As explain above, theidentifiability in UV and NIR where affected by the close proximity tothe green grass covered field and would expect to be reduced at highaltitudes.

Unlike Comparative Example 5, the solar glint from Inventive Example 3was indistinguishable from glare. The combined glint and glare increasethe reflected intensity by about 0.07 for UVA, 0.11 for RGB, and 0.15for NIR. The low specular reflectance overcoat prevented glint and glarefrom contribution to the detection signal until the target is closeenough or magnified enough for the glint and glare fill enough pixels toexceed the MTF of the imaging system.

TABLE 9B Measured target detection and identification characteristicsfor Inventive Example 3. Spectral Range UV VIS-B VIS-G VIS-R NIR SkyMax-Min 0.04 0.12 0.11 0.11 0.13 Example 3 Detectability 0.14 0.11 0.020.00 0.10 Ref W Detectability 0.22 0.26 0.31 0.34 0.41 Ref KDetectability 0.37 0.46 0.26 0.12 0.02 Example 3 Identifiability 0.060.00 0.00 0.01 0.16 Ref W Identifiability 0.15 0.20 0.16 0.05 0.44 Ref KIdentifiability 0.37 0.43 0.24 0.11 0.04 Example 3 Solar Glint 0.07 0.110.11 0.11 0.15 Addition Ref W Solar Glint Addition0.30 >0.33 >0.40 >0.44 >0.28 Ref K Solar Glint Addition 0.62 0.87 0.890.90 NA

Inventive Example 4: Aerial High UV Reflectance Blue-Sky CamouflageCoating

A first coating fluid to act as a broadly absorbing underlayer wasprepared with carbon black, porous particles with pore sizes effectiveat scattering in the NIR, and a matrix polymer comprising Hystretch® V43(a polyurethane emulsion from Lubrizol with a T_(g) of −43° C.) andAcrysol™ G-111 (an ammonia neutralized poly-acrylate solution polymeruseful as viscosity modifier available from Dow Chemical Company). Theresulting fluid had 50 volume % solids plus pores and 40 weight % solidswith 1.7% carbon by weight.

A second coating fluid to emulate blue sky was made using the 5.4 micronparticle P6 at 36 volume % solids plus pores and at volume ratio of 3parts by volume particles to 2 parts matrix polymer, where the matrixpolymer included Hystretch® V43, Cycmel 373 (a partially methylatedmelamine useful as a cross-linker from Palmer Holland Inc.), andAcrysol™ G-111. A solution of direct blue 71 was added to obtain a levelof 1758 ppm in the final dry coating.

A third coating fluid to act as a low specular reflectance overcoat wasprepared using the 5.4, 12, and 30 micron particles P6, P7, and P8 in aratio of 2-to-2-to-1 at 36 volume % solids and at a volume ratio of 8parts by volume particles to 5 parts matrix polymer, where the matrixpolymer included Hystretch® V43 and Acrysol™ G-111.

An inflated spherical 16 in black Orbz balloon was spray coated with thefirst fluid comprising porous polymer particle, carbon black, and matrixpolymer to achieve sufficient absorbance in the IR. When dry, the secondcoating fluid was sprayed over the top two thirds of the balloon anddried. The balloon was coated multiple times with thin spray coats ofthe third fluid to achieve uniform low specular reflectance,corresponding to a combination of FIG. 6A and FIG. 8D. Several extracoatings of the third coating fluid were applied on the bottom third ofthe balloon to increase the reflectance in the UVA/B and VIS wavelengthranges.

The balloon was imaged in UV, VIS, and NIR spectral ranges against ablue sky at low humidity about 50 to 75 feet above a grass field inmid-fall. The balloon was sectioned and the total diffuse reflectancespectrum was measured for lightly and heavily coated areas of both thetop and bottom of the balloon. Table 10A summaries the total diffusereflectance of the balloon sections. Table 10B summarizes variousdetectability and identifiability metrics evaluated from the imagedballoons. The extra coatings of the low specular reflectance topcoatapplied to the bottom of the balloon was not enough to replace thereflectance UV and VIS-B lost due to not applying the blue-sky emulationsecond coating fluid to this part of the balloon. This increased thedetectability over that which one would achieve when the blue-skyemulation second coating fluid is applied to the whole balloon. Theincreased UV reflectance of Inventive Example 4 decreased theidentifiability in the UV wavelength range. The range in coatingthickness on the top of the balloon produced differences in reflectancethat were about equal to that of the solar glint and glare. The coatingvariations area modulations were somewhat less than the area of thesolar glint and glare on the curved surface of the balloon providingadditional camouflage protection.

TABLE 10A Measured reflectance characteristics for Inventive Example 4.% Total Diffuse Reflectance of Normal Incident Light UVB UVA VIS-B VIS-GVIS-R NIR SWIR 280- 320- 400- 500- 600- 720- 1000- 315 380 500 600 7001000 1800 Sample nm nm nm nm nm nm nm Top Lt Coat 45.0 49.6 35.2 18.413.6 10.9 7.3 Top Hv Coat 42.7 49.1 39.2 22.0 18.1 17.1 9.7 Bot. Lt Coat34.0 29.2 18.4 12.9 10.5 9.8 14.7 Bot. Hv Coat 35.9 42.8 30.0 19.7 13.810.9 11.0

TABLE 10B Measured target detection and identification characteristicsfor Inventive Example 4. Spectral Range UV VIS-B VIS-G VIS-R NIR SkyMax-Min 0.07 0.29 0.25 0.19 0.24 Example 4 Detectability 0.10 0.05 0.000.03 0.15 Ref W Detectability 0.19 0.16 0.29 0.42 0.13 Ref KDetectability 0.48 0.58 0.29 0.09 0.15 Example 4 Identifiability 0.020.00 0.03 0.06 0.23 Ref W Identifiability 0.05 0.04 0.13 0.00 0.00 Ref KIdentifiability 0.45 0.55 0.27 0.07 0.07

Inventive Example 5: Aerial Overcast-Sky Camouflage Coating

A coating fluid to emulate an overcast sky was made using particle P9 at46 volume % solids plus pores and at volume ratio of 2 parts particlesto 3 parts matrix polymer, where the matrix polymer included Hycar®26120 and Carbopol® Aqua SF-1. Samples were prepared as described inInventive Example 3 including the same low specular reflectance topcoatfluid. No subtractive colorant was used and the combined coatingscorresponding to FIG. 8D. The Mie backward scattering effective poresize was 200 nm for the combined layers. The coated balloon was imagedon a solidly overcast but bright day at 50 to 75 feet above snow. Thelighting was completely diffuse for these conditions and there was noobservable specular reflection for Inventive Example 5 and the Referenceballoons. Table 11A summaries the total diffuse reflectance of thesamples. Table 11B summarizes various detectability and identifiabilitymetrics.

TABLE 11A Measured reflectance characteristics for Inventive Example 5Total Diffuse Reflectance of Normal Incident Light % UVB UVA VIS-B VIS-GVIS-R NIR SWIR 280- 320- 400- 500- 600- 720- 1000- 315 380 500 600 7001000 1800 Sample nm nm nm nm nm nm nm 3M Vinyl 5.5 5.1 5.0 5.1 5.0 4.84.6 Blade Coating 45.2 58.1 54.1 47.3 41.3 32.3 17.9 Slot Coating 42.459.4 60.2 54.0 48.0 38.7 22.6

TABLE 11B Measured target detection and identification characteristicsfor Inventive Example 5. Spectral Range UV VIS-B VIS-G VIS-R NIR WhiteSky Max-Min 0.02 0.07 0.08 0.09 0.05 Example 5 Detectability 0.02 0.000.00 0.00 0.07 Ref W Detectability 0.19 0.05 0.05 0.04 0.07 Ref KDetectability 0.49 0.46 0.40 0.36 0.21 Example 5 Identifiability 0.020.00 0.00 0.00 0.07 Ref W Identifiability 0.17 0.04 0.04 0.03 0.06 Ref KIdentifiability 0.47 0.45 0.39 0.35 0.12

While the overcoat was penetrated by water in about 1 hour the whiteemulation layer showed no loss of light scattering for 8 hours. Thiswhite sky emulation multispectral camouflage coating would maintain itscolor performance after being exposed to rain with slow dryingconditions under heavy clouds.

Inventive Example 6: Impermeable Blue-Sky Camouflage Coating

A first barrier layer comprising 15% AQ™ 38S (a sulfopolyester having aT_(g) of 38° C. from Eastman Chemical Company) dissolved in water wascoated on CDT 3M™ Wrap Film Series 1080 matte black vinyl using a #7wire wound rod from RD Specialties Inc. to form a 2.6 micron polymerbarrier layer. This barrier layer was effective in preventing the UVstabilizer octocrylene in the vinyl from migrating into the porousparticle coatings at temperatures below about 33° C. A second barrierlayer comprising 5% 996,000 Mw PMMA dissolved in ethyl acetate wascoated over the first barrier layer using a #20 wire wound rod from RDSpecialties Inc. to form a 2.5 micron polymer barrier layer that waseffective in slowing the octocrylene migration into the porous particlecoatings at temperatures below about 100° C. thus enabling intermittentworking temperatures above 33° C. A coating fluid to form a blue-skyemulation layer comprising porous particles, matrix polymer and asubtractive colorant was made using the 5.4 micron particle P6. Thecoating fluid contained 43 volume % solids plus pores at volume ratio of2 parts particles to 3 parts matrix polymer, where the matrix polymercomprised 99% Hycar® 26120 and 1% Carbopol® Aqua SF-1 from Lubrizol. Thesubtractive colorant solution of DB 71 was added to obtain a level of353 ppm DB 71 in the final dry coating. This fluid was coated on the CTDPMMA barrier layer using #50 G wire wound rod from RD Specialties Inc.that produced a 5 mil wet coating to form a 54 micron blue-sky emulationlayer when dried corresponding to FIG. 6A.

Eight topcoat coating fluids were prepared including Porous ParticlesP6, P7, and P8 at 43 volume % solids plus pores and at volume ratio of 3parts particles to 2 parts matrix polymer, where the matrix polymercomprised Hycar® 26349 (a solvent resistant thermoset acrylic emulsionfrom Lubrizol with a T_(g) of 12° C.) and Carboset® CR675 at 0, 50%,75%, and 100% Carboset®. Dowanol™ PMA from Dow Chemicals, Inc. was as acoalescing aid, and either 0% and 0.14% dimethyl ethanol amine (DMEA) asa pH modifier. These fluids were coated using a blade having a 2 mil gapto produce coatings, corresponding to FIG. 8D, that were about twice theoptimum thickness for low specular reflectance coatings to enabletoughness studies using the micro-scratch test described above. Thecoatings were dried at 45° C. for 5 minutes then cured at 98° C. for 5minutes.

The coatings were evaluated using the micro-scratch test and wettingtest described in the Evaluations Methods section above. Additionally,the ability to be stretched at 20° C. without cracking was subjectivelyevaluated under a microscope. The thermoset acrylic emulsions arealkaline, and the cross-linking reaction requires high pH. The DMEAraises the pH to counter the acidic silica from the porous particlesthus increasing the cross-linking the Hycar® and as well as increasingthe adhesion of the Carboset® polymer resulting in greater toughness andscratch resistance. The pH modifier had no effect on wetting andpenetrations of the coatings by water. Compositions containing at least50% Carboset® CR675 gave wetting times greater than time it would taketo dry off a target encountering a brief rainstorm in blue skyconditions. The styrenic composition and high T_(g) of the Carboset®CR675 (70° C.) compared to the Hycar® 26349 (12° C.) provided greaterresistance to penetration of the porous particles by water. Observedcracking of the coatings occurred at lower levels of elongation forincreasing Carboset® CR675 in the matrix polymer. This crackingperformance is related to the T_(g) of the polymers and would occur atgreater elongations for the recommended temperature of 40° C. forapplication of the film to substrates.

TABLE 11 Scratch and wetting characteristics for Inventive Example 6Micro-Scratch Wetting Failure Load (g) Time (hours) Carboset ® CR675 noDMEA 0.14% DMEA Onset Complete   0% 125 274 0.25 0.75  50% 105 130 1 1.5 75% 109 138 1.5 2.5 100% 155 190 2.5 3.5

The present invention provides at least the preceding embodiments andcombinations thereof, but other variations and modifications areconsidered to be within the present invention as a skilled artisan wouldappreciate from the teaching of this disclosure:

PARTS LIST

-   100 article-   105 substrate-   110 surface-   120 coating-   125 porous particle-   125 a porous particle-   125 b porous particle-   125 c porous particle-   125 d porous particle-   125 e porous particle-   125 f porous particle-   130 binder-   132 colorant-   135 impermeable shell-   140 pores-   142 first set of pores-   144 second set of pores-   146 third set of pores-   150 underlying layer-   155 overcoat layer-   160 cap-   162 cap-   164 cap-   200 multimodal particle size distribution-   205 distribution function-   210 mode-   212 mode-   214 mode-   216 mode-   240 graph-   250 spectrum-   260 spectrum-   270 spectrum-   300 form suspension of monomer droplets step-   310 polymerize monomers step-   320 remove porogen step-   330 remove inorganic colloid step

1. An article comprising: a substrate with a surface; and a coatingdisposed over the surface, including: a plurality of porous polymerparticles having pores with a variety of pore sizes including a firstset of pores having a first average pore size d1 in the range0.3≤d1/λ1≤0.7, wherein λ1 is a wavelength in the range of 250-400 nm, asecond set of pores having a second average pore size d2 in the range0.3≤d2/λ2≤0.7, wherein λ2 is a wavelength in the range of 400-700 nm,and a third set of pores having a third average pore size d3 in therange 0.3≤d3/λ3≤0.7, wherein λ3 is a wavelength in the range of 700-3000nm, wherein the porous polymer particles have a shell which isimpermeable to a liquid; and a binder material.
 2. The article of claim1, wherein at least some of the porous polymer particles arepanchromatic scattering porous polymer particles, each of thepanchromatic scattering porous polymer particles including pores in thefirst set of pores, pores in the second set of pores and pores in thethird set of pores.
 3. The article of claim 2, wherein the panchromaticscattering porous polymer particles have pores with a continuum of poresizes which include pores in the first set of pores, pores in the secondset of pores, and pores in the third set of pores.
 4. The article ofclaim 1, wherein the porous polymer particles include: ultravioletscattering porous polymer particles having pores in the first set ofpores; visible light scattering porous polymer particles having pores inthe second set of pores near infrared scattering porous polymerparticles having pores in the third set of pores.
 5. The article ofclaim 1, wherein at least some of the porous polymer particles arepanchromatic scattering porous polymer particles, each of thepanchromatic scattering porous polymer particles including pores in thefirst set of pores and pores in the second set of pores, and whereinvoids between the panchromatic scattering porous polymer particlesprovide pores in the third set of pores.
 6. The article of claim 1,wherein the porous polymer particles include: ultraviolet scatteringporous polymer particles having pores in the first set of pores; andvisible light scattering porous polymer particles having pores in thesecond set of pores; wherein voids between the porous polymer particlesprovide pores in the third set of pores.
 7. The article of claim 1,wherein the substrate is a metal or has a metalized surface.
 8. Thearticle of claim 1, wherein the porous polymer particles have a particlesize between 0.5-100 μm.
 9. The article of claim 1, wherein the porouspolymer particles have a porosity in the range of 10-50%.
 10. Thearticle of claim 1, further including one or more underlying layersdisposed between the substrate and the coating.
 11. The article of claim10, wherein one of the underlying layers includes a broad-band absorbingmaterial.
 12. The article of claim 10, wherein one of the underlyinglayers includes a subtractive colorant.
 13. The article of claim 10,wherein a first underlying layer is a barrier layer including a polymerhaving a T_(g) at least 5° C. above a specified continuous operatingtemperature and a second underlying layer disposed over the firstunderlying layer is a barrier layer including a polymer having a T_(g)at least 5° C. above a specified intermittent operating temperature. 14.The article of claim 1, further including one or more overcoat layerspositioned over the coating.
 15. The article of claim 14, wherein one ofthe overcoat layers is a low-specular-reflectance surface layerincluding a plurality of protruding substantially spherical caps havinga multimodal size distribution.
 16. The article of claim 1, wherein thecoating includes a subtractive colorant in the porous polymer particlesor in the binder material.
 17. The article of claim 1, wherein thebinder material is an organic polymeric material.
 18. The article ofclaim 17, wherein the organic polymeric material has an aromaticity ofless than 10% of the coating.
 19. The article of claim 1, wherein nomore than 5% of the coating corresponds to interstitial voids not filledwith binder.
 20. The article of claim 1, wherein the article is atextile.